JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. A1, PAGES 127-139, JANUARY 1, 1997

Dynamical polar cap: A unifying approach

Patrick T. Newell, l Dingan Xu, 2 Ching-I. Meng, 1 and Margaret G. Kivelson 3

Abstract. Various polar cap configurations have been proposed as typical for northward interplanetary magnetic field conditions (IMF B z > 0), including that (1) a central region empty of plasma sheet precipitation and arcs remains even after several days of profound magnetic quiet; (2) the polar cap is routinely closed for northward IMF; (3) the polar cap is open, with arcs and polar rain interspersed; and (4) the polar cap is closed, but that arcs separated by no precipitation are spread throughout the polar cap. We have studied eight cases of B z > 0 intervals using DMSP precipitating particle data and high time resolution (15.36 s) IMF data under a variety of conditions. It turns out that all of the first three proposed configurations can occur, but each under a specific set of circumstances. When B z > 0 and B z - IBvl continuously, the polar cap maintains a central finite open region void of precipita- tion. When B z > IB•.I for about 4 hours, the polar cap completely closes, with plasma sheet precipitation across'the entire polar cap, and no significant gaps in precipitation. Because the rate at which dayside merging can open flux (-105 Wb/s) greatly exceeds the quiet time rate of flux closure (which we estimate to be -7 x 103 Wb/s), the polar cap proves to be highly dynamic, with southward turnings of about 6-7 min sufficient to balance extended intervals without merging. For this reason, it is comparatively rare that the polar cap completely closes, because even entire days with profound magnetic quiet generally include brief southward fluctuations in B z. Finally, the configuration of polar cap arcs separated from the oval by several degrees of void or polar rain is created only for Bz< 0 after a prolonged or intense interval of B > 0.

z

1. Introduction

When the interplanetary magnetic field (IMF) is southward (B z < 0), the poleward boundary of the auroral oval is rela- tively clear, and the polar cap is usually either empty or filled with a uniform diffuse polar rain consisting of superthermal electrons from the solar wind. For northward IMF the situation

is more complex and controversial. It has been reported that the polar cap is consistently closed when the IMF is northward [Troshichev et al., 1988], but it has also been reported that the polar cap never completely closes even after days of pro- nounced quiet [Meng, 1981; Makita and Meng, 1984]. Simi- larly, polar cap arcs have been reported as following a "tear- drop" or "horse-collar" shape (that is, to be excluded from a central high latitude region) [Meng, 1981; Hones et al., 1989]. Others report that polar cap arcs can be found throughout the oval, interspersed with regions of polar rain or void [Hard), et al., 1982; Troshichev and Nishida, 1992]. This latter configura- tion is closely related to the 0 aurora, which consists of a

•The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland.

2Exploration and Production Technology Department, Texaco, Houston, Texas.

3Institute of Geophysics and Planetary Physics, University of Cali- fornia, Los Angeles.

Copyright 1997 by the American Geophysical Union.

Paper number 96JA03045. 0148-0227/97/96JA-03045509.00

single bar across the middle of the polar cap surrounded by open field lines. The 0 aurora configuration has been recently reported to arise only for southward IMF after a period of pro- longed or intense northward IMF [Newell and Meng, 1995]. The Sun-aligned arcs originally form under northward IMF conditions [Valladares et al., 1994] but are not isolated from oval precipitation until the IMF tums southward.

Most proposed types of polar cap arc configurations can be summarized into two schematic systems as shown in Figure 1. The interpretation of configuration l a can vary, however, with Hardy et al. [1982] apparently interpreting polar cap arcs as occurring on open field lines while Troshichev and Nishida [1992] interpret the configuration as closed. Peterson and Shelley [1984] studied the plasma inside a 0 aurora, and found that the ions, which included O +, were unquestionably of plasma sheet origin. The 0 aurora configuration is a special in- stance of Figure la with the interpretation that the arc is on closed field lines but surrounded by open field lines.

Most existing polar cap studies have used relatively low time resolution IMF data. Recent work has shown that even

brief IMF excursions can be highly geoeffective; for example, Greenwald et al. [1990] found that 6 min after a change in IMF By reached the ionosphere, the entire radar field of view, 15 ø MLAT by 2.5 hours MLT near noon, had converted to the new convection configuration. We took advantage of recent progress in understanding the geophysical source of particle precipitation, and the current appreciation for the importance of dynamical considerations (and thus of using high time resolu- tion IMF data) to investigate eight cases, few enough for de- tailed analysis, but numerous enough to be indicative. These cases were chosen in different ways and represent different cir- cumstances. Three cases in which the IMF was continuously

127

128 NEWELL ET AL.: DYNAMICAL POLAR CAP--UNIFYING APPROACH

(a) 12 12 O (b) Plasma

eet

24 24

Figure 1. Two common views of the polar cap under north- ward IMF conditions with respect to polar cap arcs. (a) Auroral arcs occur anywhere in the polar cap, separated by intervals of either no precipitation or polar rain. (b) Auroral arcs are ex- cluded from a central high-latitude region, constituting the re- maining open geomagnetic flux.

northward were selected (from 1986, a year for which hard copy survey plots were incidentally available); one case was chosen because AE was near 0 for 17 hours; two cases were

events previously chosen by Fairfield et al. [1996] from Geotail observations of a possibly closed magnetosphere, and two cases were selected by Xu [1995], on the basis of the pres- ence of intense energetic polar rain (the unique value of intense energetic polar rain events is the ease of identifying open field lines). Table 1 gives the eight cases selected. These four meth- ods of selecting events proved to collectively illuminate the various types of polar cap configurations based on particle pre- cipitation observations. As a result we are able to present a model which reconciles various observations of polar cap dynamics.

2. Data Presentation

The DMSP satellites are in Sun-synchronous, nearly circular polar orbits at about 835 km altitude, with orbital inclinations of 98.7 deg. The SSJ/4 instrumental package included on all

Table 1. Polar Cap Contraction Events Studied

Event Date and Time Description

Isolated Isolated

Minimum Arcs Arcs

Polar Cap Horse for After Diameter Colar B > 0 B < 0

2300 UT on April 5 to 0330 UT on April 6, 1986.

-2130 UT on May 24 to 0100 UT on May 25, 1986

3 0800 to 1800 UT on

Sept. 10, 1986

4 March 29, 1993

5 July 7, 1993

6 May 3, 1984

7 Nov. 25, 1988

1500 UT on March 13

to 1000 UT March 14, 1985

continuous B > 0, partly B z > IB¾1, partly B:- IB¾1

continuous B. > 0, mostly B > lB I, but partly B ~ lB I Z v Z 3'

mostly B z > 0, brief B. < 0, at 1030 UT; partly

B z > IB,.I, partly B.- IBI

B > 0 with interval of

B >> lB I' Geotail observa-

tions suggest closure

B > 0 with long interval of B >> lB I' Geotail observa-

z

tions suggest closure

energetic polar rain case;

mainly B z > 0, including B >>lBIbut6minB <0

z y z

excursion at 0754 UT

opened cap

energetic polar rain case; B >O, butIB/Bl>> 1. d y Z osely resembles B < 0 z

AE- 0 for 19 hours; polar cap closed(?) but re-opened at 0245 UT without AE

activity

-8 ø yes no

>-6.6 ø yes no

>-7.5 ø yes no

closed yes no 1700-1900

UT

closed

1400-1500

UT

closed

0430-0610

UT

14 ø

closed(?) (<1.7 ø) first 2

hours of

March 14

yes no

yes no

N/A no

usually no

yes,

lasting >1 hour

some

isolation, not a

clear 0

no

no

no

no

no

yes

NEWELL ET AL.: DYNAMICAL POLAR CAP--UNIFYING APPROACH 129

recent DMSP satellites uses curved plate electrostatic analyzers to measure electrons and ions from 32 eV to 30 keV with one

complete 19-point spectrum obtained each second [Hardy et al., 1984]. The satellites are three-axis stabilized, and the detector apertures always point toward local zenith. At the latitudes of interest herein, this means that only highly field-aligned par- ticles well within the atmospheric loss cone are observed. High time resolution (15.36 s) IMF data from the IMP 8 satellite was obtained from Goddard National Space Science Data Cen- ter World Wide Web server managed by Joe King.

2.1. Continuously Northward IMF (Excluding Prolonged Intervals of B z >> IByl)

Of this group of three events, event 1 will be presented as representative. From about 2300 UT on April 5, 1986, through 0330 UT on April 6, B z was positive (excepting two isolated 15.36-s samples), as illustrated in Figure 2. The (x,y,z) GSM coordinates of the IMP 8 satellite at the start of the plot, in Rœ, are given atop the plot. A DMSP F7 crossing at 2225 UT found an empty polar cap with boundaries at coordinates (0223, 73.5)/(0623, 72.0) (polar cap crossings are presented as (MLT, MLAT) coordinate pairs). Therefore prior to the contrac- tion associated with northward IMF, the polar cap radius was at nominal values, --34 deg across. By 0100 UT on April 6, the polar cap radius had shrunk to where the F6 crossings were (1938,-85.1)/(0513,-83.6), still closely resembling Figure lb, the horse-collar configuration. The minimum polar cap size seen for the entire interval was the F6 pass at 0150 UT, for which the crossings (0142, 86.1)/(2242, 86.3) were observed, giving an apparent diameter of between 7 and 8 deg, but still with a void central region.

Notice that from 0100 to 0230 UT, B z gradually declined in magnitude, from about 6 nT to less than 2 nT. After 0150 UT the polar cap ceased to contract, and as the decline in magni- tude of B z continued, the polar cap began to modestly expand again, although B z remained steadily northward (excepting

only a single 15.36-s measurement). Plate 1 illustrates the polar cap at 0233-0247 UT. The polar cap boundaries are at (0011, - 85.1)/(0343, - 82.1). Plate 1 shows that several hours into the northward IMF interval the observations are still con-

sistent with a central region nearly void of precipitation (an ex- tremely faint polar rain signature can be extracted by averag- ing). The expansion of the plasma sheet in from the dawn and dusk sides is bursty, but the bursts are embedded in a back- ground of plasma sheet precipitation. The apparent modest wid- ening of the oval in conjunction with the decline in magnitude of B z suggests that the polar cap size is dynamic rather than uniformly declining, even under quiet conditions.

Starting around 0510 UT, after the IMF had turned south- ward, arcs began to appear significantly inside the auroral oval (i.e., plasma precipitation separated by several degrees from the rest of the oval). Initially, these isolated plasma sheet fragments included downward field-aligned electron acceleration events as evinced by monoenergetic peaks. Plate 2 illustrates that these isolated arcs had faded by 0554-0603 UT, but that the associ- ated isolated plasma sheet fragments still remained (Rodriguez et al. [1995] report that polar cap arcs fade within 1 hour of a southward turning, which appears to be usually but not always the case). The configuration observed in Plate 2 closely re- sembles proposed configuration Figure l a, but note that the IMF conditions are that B z < 0 after a prolonged intense inter- val of B z > 0. Thus a • aurora or multiple isolated arc situation advocated by many did occur in this case but only after the IMF turned southward.

In none of the three cases of continuously northward IMF did we observe any configuration other than the horse-collar type. Well-isolated arcs or plasma sheet fragments (that is, separated from the rest of the plasma sheet by more than a few degrees) occurred only for event 1, as just described; although in event 2 a partially isolated arc (a few degrees) occurred after a southward turning. Thus the cases for which B z --IB•.l > 0 all maintained a central region void of precipitation, and resembled Figure lb.

20

10

0

1986/95 (-28, 19, -3) rho = 20

ß

-lO

-2o

2o

lO

o

-lO

-20

lO

5 '•;•/•t' ¾•'• o

-lO

23

, ! ,

,

96/0 1 2 3 4 5 6 DOY/UT

Figure 2. The IMF data from IMP 8 for event 1 (2300 UT on April 5, 1986, until 0600 UT on April 6). Isolated plasma sheet fragments were observed by DMSP only after 0510 UT. Note that IMP 8 ob- servations lag the IMF striking the magnetopause by --9 min.

130 NEWELL ET AL.: DYNAMICAL POLAR CAPreUNIFYING APPROACH

F6

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LOG JE 9 4-

LOG E AVE .

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I pol cap UT 02:32:55 02:•:54 02:36:54 02:38:54 02:40:54 02:42:54 02:•:54 02:46:54 MLAT -66.4 -72.9 -79.1 -84.2 -83.8 -?8.8 -?3.0 -67.0 GLAT -68.9 -75.0 -?9.7 -80.9 -77.4 -71.8 -65.4 -58.? GLONG 213.9 201.? 176.? 133.6 99.6 82.8 74.0 68.5 MLT 19:03 19:23 20:04 22:13 02:41 04:33 05:15 05:36

86/96

LOG E FLUX

ELEC ION 10 8

5 'L 3

Apr 6

Plate 1. A DMSP F6 pass on April 6, 1986, at 0233-0247 UT illustrating the typical configuration ob- served during intervals of continuously northward IMF (B,.- B,,). Plotted are differential energy flux over the range 32 eV to 30 keV for electrons and ions in unit• of log•0(eV/cm • s str eV) (main panels); aver- age energy in eV (bottom line plot) and integral energy flux in eV/cm • s str.

2.2. Geotail Cases: B z > IByl In the cases investigated in section 2.1 we saw that even

many hours of northward IMF with B z comparable to, and at times larger than, I•.l did not result in the total closing of the polar cap. However, some recent MHD simulations [e.g., Fedder and Lyon, 1995] and some recent magnetotail observa- tions [Fairfield, 1993; Fairfield et al., 1996] indicate that when B z > IByl for several hours, the magnetotail can close com- pletely, or nearly so. We therefore investigated the two cases of a closed or nearly closed magnetotail reported by Fairfield et al. [1996] from Geotail observations.

These two cases also closely resemble each other, and we present the first to represent both. The IMF data for event 4, March 29, 1993, is plotted in Figure 3. It can be seen that B z was positive from 1130 UT until -2145 UT, with B z > IByl from about 1300 until 1830 UT. Prior to the northward turning, the polar cap was wide open (>30 ø diameter), while by 1344 UT the polar cap had contracted to such a state that F11 crossed boundaries at (1339, 87.4)/(0634, 85.9) (an F10 crossing

at about the same time in the southern hemisphere put the boundary at 0115 MLT as -85.8 MLAT). Throughout this in- terval, a horse-collar configuration was maintained with an empty polar cap void of arcs or plasma sheet fragments. This remained true at 1532 UT when DMSP F10 measured bound-

aries of (1209,-88.1)/(1855,-86.9), implying a tiny but finite and empty polar cap. However, by 1705 (after 4 hours of B z > I•.l) F11 data showed the polar cap closed to the highest latitude reached by the satellite, namely 89.6 MLAT. To illus- trate this complete closure of the polar cap, we present a DMSP F10 pass at 1802-1811 UT in Plate 3. (This later F10 pass shows more intense and thus more striking plasma sheet fluxes in the polar cap. An example of a recently closed polar cap is given in section 2.3). The highest latitude reached by F10 on this pass is 89.2 MLAT, and plasma sheet precipitation is observed throughout the pass. There is neither a central re- gion void of precipitation nor isolated bursts of precipitation; instead the plasma sheet extends through the entire polar cap at all angles crossed by DMSP satellites (and in both hemi- spheres). This agrees fairly well with the Geotail timeline for

NEWELL ET AL.: DYNAMICAL POLAR CAP--UNIFYING APPROACH 131

F6

ELECTRONS LOG JE

LOG E AVE

12

•3 ' I 1.$• ! , , ' •' '• -' tll I 11 , lt!j I I I ,lll l , 4 '"" '""'[ '" t' "'""' "'11 i,,, ,,, "ov• "'""'T'"'"'" "" ......... ""

UT 05:54:• 05:55:25 05:56:51 05:58:16 05:59:42 •:01:07 06:02:33 06:03:59 MLAT -71.8 -7•.• -81.7 -85.4 -85.3 -81.7 -77.5 -73.4 GLAT -63.7 -68.4 -72.9 -76.9 -79.9 -81.1 -79.6 -76.3 GLONG 169.0 1•4.0 15•.3 lg.6 123.5 93.2 63.5 •.4 MLT 19:07 19:28 20:13 22:19 02:36 M:26 05:06 05:25

IONS

86/96

LOG E FLUX

ELEC ION 10 8

5 _3

Apr 6

Plate 2. The configuration observed by DMSP F6 at 0554-0604 UT for the same event as Plate 1, but after IMF turned southward.

15

10

5

0

1993/88 (32,-16,-16) rho = 23

..•,.•••-**•.••. ,,.• •. v ..,.. e ':• •'• N.. -5

-10 -15

15

10

5

o

-5

-10 -15

5 •' ß :':. 'k :;•

-lO •1 ß -15 , . .

10 12 14 16 18 20 22 0 UT

Figure 3. IMF data for March 29, 1993, an interval selected by Geotail observations as a candidate for polar cap closure.

132 NEWELL ET AL.' DYNAMICAL POLAR CAP--UNIFYING APPROACH

F10 13

ELECTRONS

LOG JE

9

MLAT GLAT GLONG MLT

12

IONS

•3 t h ' tllii i.," i! P t i.! 'I t• •!I II •., t i I l t lit I ! I t • t t t :tiI t t il Itt I !tt

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Closed: No Po]• Cap 18:02:00 18:03:17 18:04:34 18:05:51 18:07:08 18:08:25 18:09:42

71.2 75.6 79.8 84.1 88.3 87.0 82.7 73.1 76.9 79.8 81.3 80.5 77.8 74.3 17.2 6.2 348.1 320.6 291.6 270.9 258.3

20:32 20:30 20:24 20:13 18:56 09:52 09:14

18:10:59 78.2 70.3

250.4 09:07

93/88

LOG E FLUX

ELEC ION 10 8

5 _3

Mar 29

Plate 3. A DMSP F10 pass from 1802 to 1811 UT on March 29, 1993, showing a completely closed polar cap (B•. > IB,.I).

possible closures of the magnetotail suggested by Fairfield et al. [1996].

There is an ambiguous indication that by 2030 UT, after the end of the B z > IB.,.I period but before a southward turning, the polar cap slightly reopened. Unfortunately, the DMSP F11 pass at this time cannot be cleanly classified. The first unambigu- ously open polar cap (which included mantle and polar rain signatures) occurred at 2315 UT, a half hour after the south- ward turning.

The second case originally selected from Geotail observa- tions, namely July 7, 1993, also resulted in a closed polar cap (at 1400 MLT), apparently after 4 hours of B z > lB,J, although in this case an IMF data gap occurs within this 4-hour interval. Because of its close similarity to the event just presented, the July 7, 1993, Geotail case will not be presented in detail.

2.3. Polar Rain Cases

Xu [1995] found two cases of intense and energetic polar rain during prolonged intervals of northward IMF. The mason to seek such cases is that the intense polar rain signature makes it simple to identify open field lines. None of the other

cases presented here showed bright polar rain (meaning an en- ergy flux greater than 0.005 ergs/cm 2 s) under northward IMF conditions, and in our experience a clear polar rain signature implies that the IMF is southward. However, there are pub- lished cases associating polar cap arcs with northward IMF and polar rain [e.g., Hardy eta!., 1982]. Therefore we reexamined the two Xu [1995] cases (events 6 and 7) using high time reso- lution IMF data to understand instances of polar rain for north- ward IMF.

One of these polar rain events (November 25, 1988) is

readily understandable. The magnitude of B:. was steady at about -11 nT, while B z ranged between just above zero and a few nanoteslas. Under such conditions, rapid dayside merging is to be expected; indeed, substorms have been observed under IB,.I >> B z > 0 conditions. For most of the interval the polar cap was wide, near nominal values, although it did contract to 14 ø at 0830 UT as B z reached a few nanotesla positive. This event, in which a wide polar cap is filled with intense polar rain, can be interpreted as adding to the body of evidence that events with IB.,.I >> B z > 0 behave essentially the same as B z < 0 cases.

NEWELL ET AL.: DYNAMICAL POLAR CAP--UNIFYING APPROACH 133

The other event, from May 3, 1984, is even more revealing (it is not clear when IMF turned northward, but the polar cap had been contracted for a long time prior to the first IMF data, around 0600 UT). In this event the hourly average values of B z are significantly larger than B,., and even the 5-min averages available from the UCLA online data base do not indicate any southward IMF. Figure 4 illustrates the high time resolution IMF data. Plate 4 shows that as of 0833-0846 UT, what ap- pears to be a long interval of strongly northward IMF, the po- lar cap was clearly open, with intense polar rain in the south- ern hemisphere polar cap only.

A closer look at this event resolves the apparent contradic- tion with the Geotail-identified events in which the polar cap did close. Plate 5 illustrates that as of 0505-0519 UT (when

IBz/Byl ratio was largest) the polar cap was actually closed for this event also, with plasma sheet ions and erratic electrons rather than the keV polar rain present in the polar cap. An F6 pass in the northern hemisphere at the same time further con- firms that the polar cap had indeed closed. What happened be- tween Plate 5 at 0505 UT and Plate 4 at 0833 UT?

Starting at 0754 UT there was a 2-min interval of either B z < 0, or IBzl << IByl. More crucially, B z went negative at 0816 UT and apparently remained that way for slightly more than 4 min (although this interval includes a data gap). Less certainly, there is a data gap of several minutes' duration which terminates with 30 s of By >> B z (B.,. =-8 nT, B z = 2 nT). Al- lowing for a time lag of about 10 min, the half hour preceding the Plate 4 crossing included at least 6.5 min (and possibly sig- nificantly longer) of conditions ideal for rapid dayside merging. Straightforward calculations [Newell and Sibeck, 1993] show that this is enough time to open about -800,000 km 2, or to cre- ate a polar cap of diameter around 9 ø. In fact, the polar cap is somewhat larger than this in Plate 4, but the discrepancies are not so large as to be alarming, given the many uncertainties in- volved. In this example, as in event 1, the polar cap is highly dynamic, responding to IMF fluctuations on the timescale of minutes.

2.4. Prolonged Interval of AE--0

The final event is selected using the approach often previ- ously adopted to search for the ground state of the magneto- sphere; namely a prolonged interval of near-zero AE is studied. For about 17 hours, from 1500 UT on March 13, 1985, through 1000 UT on March 14, 1985, AE remained below about 25 nT as shown in Figure 5. Thereafter the AE slowly rose to about 100 nT by 1100 UT. The only IMF information available in the key period is a few minutes of southward IMF just after 0800 UT.

The polar cap demonstrated dynamic behavior not predict- able from the continuously low values of AE. It did not simply slowly contract to a minimum value and remain there. Initially, the cap did contract in a straightforward manner, with the polar cap radius declining to less than 4 deg by the first hour of March 14. (Two F6 passes, one in the southern hemisphere around 0015 UT and one in the northern around 0100 UT, sug- gest a closed polar cap, but the highest latitude reached in the first case is only -88.3 MLAT, and in the second pass 85.0 MLAT). However, without any change in the very low value of AE, the polar cap thereafter modestly opened again, up to several degrees by 0440 UT on March 14 when F7 had cross- ings of (0509, 83.9)/(0132, 83.2). Plate 6 shows an illustrative F6 crossing at about the same time (0505-0519 UT on March 14, 1985); it is apparent that a central region void of either po- lar arcs or polar rain is maintained. The expansion of plasma in along the dawn and dusk flanks is bursty, but continuous.

However, after AE began to increase, an arc did begin to de- tach from the oval, becoming first apparent in an F6 crossing at 1110 UT. This region containing nightside plasma far detached from the oval remained for about 2 hours after AE began to in- crease, although signs of field-aligned electron acceleration did not persist that long. Plate 7 shows that even at 1244-1300 UT, the detachment persisted, long after magnetic activity com- menced. Thus the global configuration resembled Figure lb un-

2O

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x m 0

-10

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15

10

5

m 0

-5

-10 -15

lO

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m 0

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6

1984/124 (-18, -27, -10) rho = 28

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.... ._ ..... ::'<' .' . ß ß

....... ' '%'('t•"•¾'•' 1 9 10 11

UT

Figure 4. IMF data for May 3, 1984, in an interval in which the polar cap apparently remained open during strongly northward IMF.

134 NEWELL ET AL.' DYNAMICAL POLAR CAP--UNIFYING APPROACH

F6

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UT 08:33:00 08:34:51 08:36:42 08:38:33 08:40:25 08:42:16 08:44:07 08:45:59 ML^T -69.1 -75.6 -82.0 -86.6 -83.5 -72.9 -72.5 -67.1 GLAT -54.4 -60.6 -66.8 -72.6 -77.8 -80.8 -79.9 -75.6 GLONG 134.0 130.0 124.4 115.3 98.2 65.4 25.6 0.4 MLT 17:54 17:37 16:52 13:22 08:31 07:29 07:03 06:47

12

IONS

84/124

LOG E FLUX

ELEC ION 10 8

5 3

May 3

Plate 4. A DMSP F6 pass at 0833-0846 UT on May 3, 1984, showing an open polar cap during an interval in which the overall sense of IMF was strongly northward, yet the polar cap is observed to be open. This event is during a rare period of keV polar rain (corresponding to a solar proton event), thus making it especially easy to identify the polar rain.

til after the increase in AE began, whereupon the configuration did resemble Figure la (with a single detachment).

3. Discussion

3.1. Polar Cap Arcs and Polar Rain

In the eight examples we studied, and in our general experi- ence, bright polar rain signatures (>0.005 ergs/cm 2 s) were ob- served only after intervals appropriate to rapid dayside merging (B z < 0 or Bz << IB•.l). It does not seem to be necessary that the IMF is exclusively southward or even averages southward; it is enough that substantial portions of the previous half-hour (after appropriate time lagging) contain IMF conditions appropriate to rapid dayside merging. However, Hardy et al. [1982] reported one interval of polar cap arcs apparently interspersed with polar rain. This occurred during a prolonged interval of strongly northward hourly average values for Bz---from 0600 to 1100 UT on December 12, 1977. The two passes which show

polar rain intermixed with auroral arcs occur at 0653-0718 UT and 0745-0810 UT.

High time resolution IMF data for December 12, 1977, is shown in Figure 6. At this time IMP 8 was located around x = 25Rœ, y = 19Rœ, with a time lag of about 10 min being ap- propriate to the following discussion (times given are not lagged). Prior to the first pass (around 0700 UT) with mixed polar rain and arcs there was an intense southward turning reaching -5 nT and lasting many minutes. The IMF measured by IMP 8 then resumed northward, with conditions unfavorable to dayside merging until 0725:41 UT. At this time, B z began fluctuating about zero while B•. varied between about 5.5 and 7.0 nT. This period favorable to dayside merging lasted until 0731:34 UT, about 6 min. When the time lag is also factored in, it is easy to understand why polar rain would be present at the time concentrated upon by Hardy et al. [1982], namely 0745-0810 UT.

We now argue from theoretical grounds that polar rain at moderate or higher intensity is to be expected only when rapid dayside merging has occurred recently. Dayside field lines that

NEWELL ET AL.: DYNAMICAL POLAR CAP--UNIFYING APPROACH 135

F7

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IONS

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lu-tl ,, •' I11{,tl '4 Closed: No Polar Cap

UT 05:05:00 05:06:59 05:08:59 05:10:59 05:12:59 05:14:59 05:16:59 05:18:59 MLAT -64.3 -70.8 -77.3 -83.7 -87.1 -82.0 -75.8 -69.4 GLAT -70.2 -76.1 -80.3 -80.4 -76.4 -70.5 -64.1 -57.4 GLONG 231.1 217.3 188.5 144.6 114.5 100.0 92.1 86.9 MLT 22:31 22:40 22:55 23:45 05:21 08:49 09:26 09:42

84/124

LOG E FLUX

ELEC ION 10 8

5

May 3

Plate 5. A DMSP F7 pass for the same event as Plate 4, except at 0505-0519 UT, prior to a brief southward IMF excursion. This pass shows a closed polar cap with no polar rain (the ions are well above noise level throughout the polar cap).

are opened within the last few minutes contain intense magnetosheath plasma with accelerated ions, forming the par- ticle cusp. As the high-altitude end sweeps downstream, ions which cross the magnetopause current layer tailward of the magnetic cusp are de-energized [e.g., Sonnerup, 1984; Wing et al., 1996], and those directed earthward must also have a ther- mal velocity opposed to the tailward-directed magnetosheath flow and can thus form only a weak low-altitude mantle signa- ture. Charge quasi-neutrality similarly reduces the electron en- try. Further downstream so few ions enter that their flux is not easily detectable with DMSP, although a modest flux of elec- trons is still detectable, forming the polar rain (the light elec- tron mass means electron fluxes are much higher than ion fluxes for equal densities). However, as the high-altitude end continues to sweep downstream, almost no ions enter, and even the polar rain becomes weak. This is the likely explanation for the negative noon-midnight gradient in the polar rain [Torbert et al., 1981; Gussenhoven et al., 1984].

Consider now the motion of the low-altitude footprint of the opened field lines away from the merging gap, which mo- tion depends primarily on the subsequent rate of merging

[Siscoe and Huang, 1985; Cowley and Lockwood, 1992]. When merging is going briskly, the ionospheric footprint can move across the entir(• polar cap before the high-altitude end convects too far downstream; hence the polar cap can fill with intense polar rain (providing the solar wind superthermal elec- tron population is suitably intense). However, when flow through the merging gap is slow, bright polar rain can be found only near the merging site.

Experimental evidence for the association of moderate to in- tense polar rain exclusively with rapid dayside merging situa- tions will be discussed elsewhere (T. Sotirelis et al., Polar rain as a diagnostic of rapid dayside merging, submitted to Journal of Geophysical Research, 1996). For the present, it is enough to note that for the examples we have studied ourselves, and for those found in the literature, polar rain occurs only when intervals of B z < 0 (or IB•. I>> B z > 0) are associated.

3.2. Timescales for Polar Cap Dynamics

Previous studies looking for a closed or "baseline" magneto- sphere have concentrated on intervals of tens of hours or even days of profound magnetic quiet. It is now clear that the polar

136 NEWELL ET AL.: DYNAMICAL POLAR CAPMUNIFYING APPROACH

U.l

1000

800

600

400

200

85/72 cap is too dynamic for this to be an adequate approach. During these apparently quiet intervals, the polar cap can deflate over a period of a few hours, then significantly re-inflate after a period of as little 5-10 rain of southward IMF, or even IB).I >> B z > 0.

On the other hand, in the unusual case where B z > IByl con- tinuously for several hours (4 hours in our examples), the polar cap can completely close. Our study confirms that this is true of both the events suggested by Fairfield et al. [1996]. Such events can be used to estimate the quiet time rate of flux re- moval from the polar cap. The Geotail event discussed in sec- tion 2.2 showed a polar cap reduction from a radius of 6.7 ø at 1340 UT to closed by 1700 UT. This gives a deflation rate of -7 x 10 3 Wb/s, a value which seems to be crudely constant in this interval (i..e., taking the intermediate crossings around 1500 UT gives the same result). Of course, it is known that substorm activity can reconnect flux much faster than this [Angelopoulos et al., 1996], but this rate is occurring during a very quiet time, and seems to imply a modest level of "back- ground" merging on the nightside.

Figure 5. AE data for an interval of prolonged quiet, in hours, from 0000 UT on March 13, 1985. A 0 aurora was first ob- served at 1110 UT (about hour 35); the detached plasma sheet fragment shown in Plate 7 occurs at hour 37.

F6

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• 05:11:• 05:12:51 05:14:42 05:16:33 05:18:25 05:20:16 05:22:07 05:23:59 MLAT -66.8 -73.3 -79.4 -84.3 -•.0 -79.3 -74.1 -69.1 GLAT -61.1 -67.3 -73.0 -78.1 -80.9 -79.7 -75.4 -69.8 GLONG 181.3 175.5 166.1 148.3 114.2 75.0 51.1 38.9 MLT 18:56 19:18 20:01 22:14 02:37 •:23 05:01 05:21

85/73

LOG E FLUX

ELEC ION 10 8

Mar 14

Plate 6. An F6 crossing on 0511-0524 UT on March 14, 1985, showing that during continuously posi- tive B z, a central region void of either polar arcs or polar rain is maintained.

NEWELL ET AL.: DYNAMICAL POLAR CAP--UNIFYING APPROACH 137

F6

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85/73

LOG E FLUX

ELEC ION 10 8

5 3

Mar 14

Plate 7. An isolated plasma sheet fragment in the polar cap, observed by F6 at 1245-1300 UT March 14, 1985, about 2 hours after magnetic quiet gave way to rising AE activity.

lO

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o

-5

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1977/,346

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6 7 8 9 10 11

UT

Figure 6. The IMF data from IMP 8 (x = 25RE, y = 19RE) for the interval wherein Hardy et al. [1982] found auroral arcs mixed with polar rain.

138 NEWELL ET AL.' DYNAMICAL POLAR CAP--UNIFYING APPROACH

In contrast to this slow background removal of existing open flux, a 100-kV potential across the merging gap opens new flux at the rate of 105 Wb/s [Newell and Sibeck, 1993]. This ex- plains the dynamic behavior of the polar cap: even an interval on the order of 6-7 min of rapid dayside merging is enough to profoundly alter the balance of flux in the polar cap. No matter how intensely northward the average value of IMF is during a quiet interval, dayside merging cannot proceed at a slower rate than zero; yet a brief embedded interval of rapid merging can to a significant extent re-inflate the polar cap.

This line of reasoning shows how hard it is for the polar cap to close completely, and how difficult it is to stay closed. No matter how large and positive the average value of B z is during a quiet interval, a fluctuation in the value of B z of a few minutes duration can pump new open flux into the polar cap (without instantly reconfiguring it, of course). Five or six min- utes of such merging can require about an hour of background reconnection to be removed (excepting enhanced nightside ac- tivity such as a bursty bulk flow, of course). Although the hourly average value of B z is fairly often stably positive for several hours, for most such cases the B•. component fluctuates when observed at higher time resolution. Thus often even a prolonged interval of northward IMF contains enough embed- ded brief spurts of dayside merging to keep the polar cap open and finite.

4. Conclusions

Many different types of possible polar cap configurations have been reported in the literature, and examples of most of those suggested can indeed be found in our survey. These dif- ferent configurations are not randomly occurring state functions of a quiet magnetosphere, but arise in specific situations, as a result of the dynamic behavior of the polar cap, resolvable by resort to the high time resolution IMF data. Table 2 summa- rizes the different types of polar cap configurations observed in the DMSP data set of particle precipitation observations, and the conditions which give rise to them.

The horse-collar or tear-drop configuration originally pro- posed by Meng [1981], Hones et al. [1989], and others, with a central open region void of polar cap arcs and magnetotail plasma occurs when B z is continuously northward, but with IByl- B z (or at least not several continuous hours of B z > IByl).

The polar cap can completely close, as suggested by Troshichev et al. [1988], but only for events such as those se- lected by Fairfield et al. [1996], namely those with B z > IByl for several hours, without even brief excursions into rapid merging conditions. In this case, arcs (but neither void nor polar rain) can be found anywhere throughout the polar cap, which is filled by the plasma sheet. However, because the rate at which

new flux can be opened on the dayside (-105 Wb/s) greatly ex- ceeds the quiet time removal of open flux (estimated from our observations at -7 x 103 Wb/s), even relatively short fluctua- tions of B z to negative or near-zero values can keep the polar cap open during long intervals with average values of B z that are large and positive. For this reason, the ground state of the magnetosphere cannot be found by examining intervals of pro- found magnetic quiet lasting days or more; instead, it is neces- sary to examine the IMF data at a high time resolution.

The counterintuitive configuration implied by the 0 aurora [Frank et al., 1982] can exist, with polar rain or void surround- ing both sides of one or more polar cap arcs containing nightside plasma sheet precipitation. To date observations show that this odd configuration arises when the IMF B z tums southward after a prolonged interval of northward IMF, both in the cases studied here and elsewhere [Newell and Meng, 1995]. There are no known cases in which a satellite near the Earth-Sun line ob-

served a continuously northward IMF while a low-altitude space- craft observed polar rain separating a polar cap arc from the main oval. Table 2 summarizes the polar cap configuration under various circumstances.

Acknowledgments. The DMSP SSJ/4 detectors were designed and built by D. Hardy and colleagues. The high-resolution IMP 8 data were obtained from J. King's NSSDC WWW data distribution site (R. Lepping, P.I.). Research at APL was funded by AFOSR grant F49620-92-J-0196. Work by Xu and Kivelson was supported by NSF grant ATM93-14239.

The Editor thanks W. K. Peterson and another referee for

their assistance in evaluating this paper.

References

Angelopoulos, V., et al., Multipoint analysis of a bursty bulk flow event on April 11, 1985, J. Geophys. Res., 101, 4967, 1996.

Cowley, S. W. H., and M. Lockwood, Excitation and decay of solar wind-driven flows in the magnetosphere- ionosphere system, Ann. Geophys., 10, 103, 1992.

Fairfield, D. H., Solar wind control of the distant magnetotail: ISEE 3, J. Geophys. Res., 98, 21,265, 1993.

Fairfield, D. H., R. P. Lepping, L. A. Frank, K. L. Ackerson, W. R. Paterson, S. Kokubun, T. Yamamoto, K. Tsuruda, and M. Nakamura, Geotail observations of an unusual magnetotail under very northward IMF conditions, J. Geomagn. Geoelectr., 48, 473-487, 1996.

Fedder, J. A., and J. G. Lyon, The Earth's magnetosphere is 165 Re long: Self-consistent currents, convection, magneto- spheric structure and processes for northward interplan- etary magnetic field, J. Geophys. Res., 100, 3623, 1995.

Frank, L. A., J. D. Craven, J. L. Burch, and J. D. Winningham, Polar views of the Earth's aurora with Dy- namics Explorer, Geophys. Res. Lett., 9, 1001, 1982.

Table 2. Polar Cap Configurations and When They Occur

Configuration Conditions of Occurrence

Wide polar cap, and/or clear polar rain (- 0.005 ergs/cm 2 s)

Polar cap - 10ø-20 ø diameter or smaller, central region void except for faint polar rain

Polar rain and arcs interspersed

Polar cap closed

B <0orlBl>>B >0 z 3' z

B >0, IBI- IBI z z y

B < 0 after long or intense interval of B > 0 z z

B > lB I for >-4 hours without B < 0 fluctuations z 3' z

NEWELL ET AL.: DYNAMICAL POLAR CAP--UNIFYING APPROACH 139

Frank, L. A., et al., The theta aurora, J. Geophys. Res., 91, 1986.

Greenwald, R. A., K. B. Baker, J. M. Ruohoniemi, J. R. Dudeney, M. Pinnock, N. Mattin, J. M. Leonard, and R. P. Lepping, Simultaneous conjugate observations of dy- namic variations in the high-latitude dayside convection due to changes in IMF By, J. Geophys. Res., 95, 8057, 1990.

Gussenhoven, M. S., D. A. Hardy, N. Heinemann, and R. K. Burkhardt, Morphology of polar rain, J. Geophys. Res., 89, 9785, 1984.

Hardy, D. A., W. J. Burke, and M. S. Gussenhoven, DMSP optical and electron measurements in the vicinity of polar cap arcs, J. Geophys. Res., 87, 2413, 1982.

Hardy, D. A., L. K. Schmitt, M. S. Gussenhoven, F. J. Marshall, H. C. Yeh, T. L. Shumaker, A. Hube, and J. Pantazis, Precipitating electron and ion detectors (SSJ/4) for the block 5D/flights 6-10 DMSP satellites: Calibration and data presentation, AFGL Tech. Rep., AFGL-TR-84- 0317, 1984.

Hones, E. W., J. D. Craven, L. A. Frank, D. S. Evans, and P. T. Newell, The horse-collar aurora: A frequent pattern of aurora in quiet times, Geophys. Res. Lett., 16, 37, 1989.

Makita, K., and C.-I. Meng, Average electron precipitation patterns and visual aurora characteristics during geomag- netic quiescence, J. Geophys. Res., 89, 2861, 1984.

Meng, C.-I., The auroral electron precipitation boundary dur- ing extremely quiet geomagnetic conditions, J. Geophys. Res., 86, 4607, 1981.

Newell, P. T., and C.-I. Meng, Intense keV energy polar rain, J. Geophys. Res., 95, 7869, 1990.

Newell, P. T., and C.-I. Meng, Creation of theta-auroras: The isolation of plasma sheet fragments in the polar cap, Sci- ence, 270, 1338, 1995.

Newell, P. T., and D. G. Sibeck, Upper limits on the contri- bution of flux transfer events to ionospheric convection, Geophys. Res. Lett., 20, 2829, 1993.

Peterson, W. K., and E. E. Shelley, Origin of the plasma in a cross polar cap auroral feature (theta aurora), J. Geophys. Res., 89, 6729, 1984.

Rodriguez, J. V., K. Fukui, and C. E. Valladares, The decay of polar cap arcs following north-to-south interplanetary magnetic field reversals, Eos, Trans. A GU, 76, Spring Meet. Suppl., S206, 1995.

Siscoe, G. L., and T. S. Huang, Polar cap inflation and defla- tion, J. Geophys. Res., 90, 543, 1985.

Sonnerup, B. U. O., Magnetic field reconnection at the mag- netopause: An overview, in Magnetic Reconnection in Space and Laborator), Plasmas, Geophys. Monogr. Ser., vol. 30, edited by E. W. Hones, p. 92, AGU, Washington, D.C., 1984.

Torbert, R. B., C. A. Cattel, F. S. Mozer, and C-I. Meng, The boundary of the polar cap and its relation to electric fields, field-aligned currents, and auroral precipitation, in Physics of Auroral Arc Formation, Geophys. Monogr. Ser., vol. 25, edited by S.-I. Akasofu and J. R. Kan, p. 143, AGU, Washington, D.C., 1981.

Troshichev, O. A., and A. Nishida, Pattern of electron and ion precipitation in northern and southern polar regions for northward interplanetary magnetic field conditions, J. Geophys. Res., 97, 8337, 1992.

Troshichev, O. A., M. G. Gusev, S. V. Nickolashkin, and V. P. Samsonov, Features of the polar cap auroras in the southern polar region, Planet. Space Sci., 36, 429, 1988.

Valladares, C. E., H. C. Carlson, and K. Fukui, Interplanetary magnetic field dependency of stable Sun-aligned polar cap arcs, J. Geophys. Res., 99, 6247, 1994.

Wing, S., P. T. Newell, and T. G. Onsager, Modeling the en- try of magnetosheath electrons into the dayside ionosphere, J. Geophys. Res., 101, 13,155, 1996.

Xu, D., Low altitude signatures of magnetospheric configura- tion and dynamics, Ph.D. thesis, Univ. of Calif., Los An- geles, 1995.

M. G. Kivelson, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095- 1567. (e-mail: mkivelson@igpp.ucla.edu)

P. T. Newell and C.-I. Meng, The Johns Hopkins Univer- sity Applied Physics Laboratory, Johns Hopkins Road, Laurel, MD 20723-6099. (e-mail: patrick'newell@jhuapl'edu; ching.meng @jhuapl.edu)

D. Xu, Exploration and Production Technology Department, Texaco, Houston, TX 77042 (e-mail: XUD@texaco.com)

(Received June 19, 1996; revised August 16, 1996; accepted October 3, 1996.)