ESWW10, Antwerpen, 2013 Cosmic ray variations caused by magnetic clouds in the interplanetary...

ESWW10, Antwerpen, 2013

Cosmic ray variations caused by magnetic clouds in the

interplanetary disturbances

A. ABUNIN,M., ABUNINA, A. BELOV, E. EROSHENKO, V. OLENEVA, V. YANKE (IZMIRAN)

A. PAPAIOANNOU,

H.MAVROMICHALAKI (Athens University)

[email protected]


What are the magnetic clouds?Burlaga,1981Barnden,1973Ivanov, 1984Sanderson,1990Zank, 1988Cane, 1993,2000Wibberenz,1998

Signatures of MCs:-enhanced MF-reduced MF variance;-abnormally low T of p;-SW velocity trend-high density-enhanced Fe/O-bidirectional e flux


Various kinds of the Magnetic Clouds

The impact of MCs on the GCR1. Data and methods;

2. MCs and FDs - Examples of different groups ;

3. Model for description of the GCR behavior;

4. Correction for the magnetospheric effect;

5. The reasons for discrepancy;

6. Conclusions

MCs are closely related to the FDsFDs arise during the expansion of partially closed magnetic structures MCs-most obvious example of such a structure

Data and method1. Catalogue of the ICME for 1996-2009, Richardson

and Cane, Solar Phys. 264 (2010) 189-237:properties of ICMEs, solar sources, relevant geomagnetic effects; only ICME with MC (WIND).

2. Database on the Forbush effects (IZMIRAN) based on the variations of density and anisotropy of 10GV

rigidity CR, obtained by GSM from the NM network. CR parameters above the atmosphere and magnetosphere, indexes of geomagnetic activity,Solar parameters.

99 events – into the set of ICME with MCs during the 23 and 24 solar cycles.

Common is the presence of the MCs. But they all are very different.

Caused phenomena: extreme and small magnetic storms (Kp~4);

CR variations: the biggest FD (~28%) and series of small FDs (<=0.5%);

In average:

AF=3.4 ± 0.4%), followed by (Apmax = 98 ± 9 (2nT), and Dstmin=-103 ± 8 nT) – moderate storm.

Only 62 of 99 events started with the shock arrival.

Compare the set with MCs with the control selection:

events of similar 5<Bm*Vm< 7.1

ICME-very short and long, fast (>1000km/s) and slow(<400 rm/s) near Earth, strong and modest IMF, B (50nT and ~8nT)


Some mean characteristics of FDs in the main and control groups

Mean (MC) Mean (nonMC)

Af 3.36±0.37 1.91±0.16 Axy_max 2.03±0.13 1.38±0.08 Az_range 2.04±0.10 1.36±0.06 Dmin -0.93±0.11 -0.44±0.03 Apmax 97.87±8.65 56.64±3.51 Dst_min -102.6±8.2 -56.5±3.8 Bm 20.21±1.07 18.01±0.33 Vm 551.5±16.5 652.2±9.1 VmBm 6.05±0.54 5.79±0.07

Af-FD magnitude (%), Axy_max – max of equatorial component of vector CR anisotropy (%); Az_ – range of changes of north-south component of CR vector anisotropy (%); Dmin – max of hourly decrement of CR density;Bm (nT) and Vm (km/s) – max IMF intensity and solar wind velocity for parent i disturbance, VmBm -normalized product of Vm and Bm

ICMEs with MCs more effectively modulate CR, creating deeper FDs with a more rapid decrease in the density and with greater magnitude of the CR anisotropy.


Example of the fast event with well pronounced minimum in the CR density inside the MC

Vector diagram and behavior of the CR density. Thin lines connect the equal time points on the vector diagram and density curve in every 6 hours. Vertical vectors correspond to the N-S anisotropy. Time passing by Earth the MC is shaded.


Example of relatively slow CME with the MCNot high SW velocity, and modest IMF.Behavior of 10 GV CR density and vector anisotropy

Forbush effects, both of two-step structure and with the main minimum of density in the MC, are typical. However, the presence of a magnetic cloud does not guarantee a two-step structure of the FD.


with alternation oflocal maxima and minima. These examples showthat the behavior of the CR density reflects not only the magnetic cloud in a whole but some features in its structureas well.

More complex behavior of the CR density


In some events in the central part of the MC not minimum but an increase of CR density was observed. We found 10 such events.Typically, the IMF intensity inside such a cloud is not of great importance.Perhaps, the regular structure of the magnetic field in the MC cloud facilitates the penetration of charged particles in its central part from the remote, weakly affected by FD areas of heliosphere.

Events with an increase of CR density inside the MC

There is a bigvariety of MCswith fast and slow velocity, high and lowintensity of the IMF,of the short and long duration.

Correspondingly,they cause FDs ofdifferent profile and depth.

Examples of FDs after different kind of MCs

Positive effectin CR inside ofMagnetic clouds.

Modeling: isotropic part of the CRLarge part of MCshave quasi-cylindricalgeometry.It is compatible with:SF arranged as thelong cylinder (fluxrope);

Observation of the SWdisturbances at Earth;

Successful attempts to model the MCs as

cylindrical formation.(e.g. Kuwabara et al., JGR 114, 2009)

For charged particles the distance is Xp=cVB/R in theLarmor (gyro) radiuses. R-particle rigidity (10GV in our case).More symmetric changes of CR density

Where a is a constant, b1 – trend coefficient, b2-reflecs the main part of the MC influence on the CR, bdd defines the contribution of a magnetosphere

Magnetospheric effect. Variations of Rc==Variations in count rate==closely correlate

with Dst. We applied this model to all 99 events of

a selection, each time defining parameters a, b1, b2, bd.

The simplest model is a parabola:

The CR density changes inside the MC -almost symmetrical picture with a minimum near the center in the majority of events. Good

agreement of the calculated and experimental data.Coincidence is observed not always. Main reasons for discrepancy:-cylinder form is not suitable (large dispersion; low cor coefficient)-depends on the number of hours which Earth spent in the MC

(changes from 6 to 64). An average duration of MC is 21±1 hrs, So, the number of points is sufficient for accurate modeling

. Example of modeling.Event on 2004.07.26.

Modeling:the example of model simulation of CR density variations in the MC. 74 events

MC>4pσ2 –not big


horizontal line - the coefficient bdm=0.0136±0.0016%/nT, averaged for the events when Earth was inside the MCs. The distribution of the points and errors agree with a standard

deviation σ. There is no dependence of bd on Dst.It means that in all the events relation of magnetospheric variations with the CR is the same, defined by the averaged bd .

Statistical results of modeling

bd

coefficient

bd coefficient VS the minimum Dst-index


b2 is the basic for cylindrical model

all negative b2 were observed in relatively weak field, and for Bmax>18 nT all b2 are positive. I.e., in cases of a strong magnetic field we always have "normal" FDs with a local minimum of CR density in the MC.

Selected 49 events whith b2/σb2>2 Only in 10 b2<0; But these events (increase of density) are real.

If b2 is small – this MC poorly influences the CR near Earth

b2


Influence of a size of MC on the CR density

From 74 events in 41 these influences<0.3%, and in 50 <0.5%. It is clear that such small effects can't almost be revealed by the data of single detector. All CME/ICME influence CR, especially when the magnetic cloud is observed at Earth. (pcor=0.70)

However effects of CME/ICME and MCs are often small and difficult for detection

pcor=0.70


Concentration of CR density extremes inside the MC (%).

Divide MC into central (25-75%) and lateral parts.Among all 39 events to the central zone gets 33, and in lateral only 6. Events with positive effect have a max in leading part of a cloud,with negative (min) tend to be grouped in tail part.

Distribution of the CR min (or, max) inside MCs


Conclusions The presence of MCs in the ICME significantly

increases the ability of this disturbance to modulate GCRs;

The larger MC and closer its source to the centre of disk the higher chances for Earth to get into MC, and both these factors increase a depth of FD;

Behavior of CRs inside the MC reflects both its properties as a whole and features of the MC structure;

If in the SW disturbance there is no MC it means:1) the source is not the CME but coronal hole; 2) MC initially was present but it lost its essential properties in result of interaction with the other structures (itsfield became more irregular);


Summary 1) In the most cases (but not in all) the behavior of CR density in

a MC at Earth can be described by simple parabolic dependence on distance measured in gyroradiuses;

2) The majority MCs modulate GCR, changing their density. As a rule, in a MC density of CR decreases, but there is a group of events (about 1/5 part) in which the CR density increases;

3) It was succeeded to receive quantitative correlation of CR density variations of the magnetospheric origin with Dst-index changes;

Even more explicitly, the MC affects the anisotropy than the density of the CR. As a rule, vector of anisotropy of CR changes abruptly when the Earth enters and leaves a cloud and it varies regularly inside the cloud.

These changes (together with data on the CR density) may provide additional information about the structure of the MC and its location relatively to Earth.


Thank

Y O U

ESWW10, Antwerpen, 2013 Cosmic ray variations caused by magnetic clouds in the interplanetary...

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