Interplanetary Coronal Mass Ejections During 1996-2007

7/28/2019 Interplanetary Coronal Mass Ejections During 1996-2007

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30TH INTERNATIONAL COSMICRAY CONFERENCE

Interplanetary Coronal Mass Ejections During 1996-2007

IAN RICHARDSON1, 2, HILARY CANE1,31Astroparticle Physics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, 20771, USA2CRESST and Department of Astronomy, University of Maryland, College Park, 20742, USA3School of Mathematics and Physics, University of Tasmania, Hobart, Tasmania, [email protected]

Abstract:Interplanetary coronal mass ejections are the interplanetary manifestations of coronal mass

ejections observed by coronagraphs near the Sun. Their associated effects include the generation ofupstream shocks which may accelerate energetic particles, short-term (Forbush) decreases in the ga-lactic cosmic ray intensity, unusual energetic particle flows, and geomagnetic storms. We summarizesome of the properties of ICMEs observed during solar cycle 23.

Introduction

Interplanetary coronal mass ejections (ICMEs),the interplanetary manifestations of coronal massejections (CMEs) at the Sun observed by corona-graphs are associated with many characteristicfeatures [1]. These characteristics, which are notnecessarily observed in every case, include: ab-normally low solar wind proton temperatures;solar wind ion charge state and compositionalanomalies; the generation of shocks upstream offast ICMEs which may be important acceleratorsof energetic particles; short-term (Forbush) de-creases in the galactic comic ray intensity; un-usual energetic particle flows, including bidirec-tional flows at keV to GeV energies; plasma wavegeneration; and the production of geomagneticstorms. ICMEs may also play a role in long-termcosmic ray modulation.

Primarily in view of their consequences on ener-getic particles, we have made several studies of

ICMEs in the vicinity of Earth and at 0.3-1 AUfrom the Sun using observations from near-Earthspacecraft and Helios 1 and 2 [e.g., 2, 3, 4]. Theseinclude [3] the compilation of a comprehensivelist of ICMEs passing the Earth since 1996, thebeginning of solar cycle 23, which we continue toupdate (the current version is available from theauthors). The ICME identifications are based on

solar wind plasma, magnetic field, and composi-tion data, and energetic particle observationswhich, in addition to providing information onparticle modulations and unusual particle flowswhich may be indicative of passing ICMEs, canhelp to associate a shock/ICME with a specificsolar event based on the intensity-time profile.

Figure 1 shows an example of solar wind plasma,magnetic field and energetic particle observations(from ACE and IMP 8) in the vicinity of ashock/ICME in the near-Earth solar wind in April2001. The green line marks passage of the shock,while the ICME is delineated by purple lines.Several ICME signatures are evident: Abnormallylow proton temperatures are indicated by theblack shading in theTp panel which denotes when

Tp


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INTERPLANETARY CORONAL MASSEJECTIONS

Figure 1: Energetic particle, and solar windplasma and magnetic field data associated with ashock and ICME in April 2001.

further discussion). The cosmic ray depressioncommencing at the shock is illustrated here by thecounting rate of the anti-coincidence guard of theGoddard particle instrument on IMP 8. The toppanel shows 0.5-4 MeV proton anisotropies in thesolar wind frame measured by this instrument.

The intensity (normalized to the maximum valuein a given 15-minute interval) is plotted vs. view-ing direction (GSE coordinates) such that parti-cles arriving from the direction of the Sun (or anapproaching shock) along the Parker spiral(~315) have higher intensities towards the top ofthe panel while sunward-flowing particles (~135)

lie below center. Black dashes are aligned with oropposite to the local magnetic field direction. Inaddition to a flow reversal from anti-solar to sun-ward close to shock passage in the particle en-hancement peaking at shock passage, bidirec-tional field-aligned particle flows can clearly beidentified within the ICME, indicative of particlestrapped on or mirroring along predominantlyclosed field lines in the ICME. This event was

Figure 2: ICME rate/solar rotation at the Earth,monthly sunspot number, fraction of ICMEs thatare magnetic clouds, mean ICME and solar windspeeds with speeds of individual ICMEs at 1 AUoverplotted, and the ICME rate/rotation at Ulys-ses, since 1996.

associated with a 1006 km/s halo CME at1230 UT on April 26 related to a 2B/M7.8 flare atN17W31.Using similar observations, we have identified~300 ICMEs that passed the Earth during 1996-mid 2007. The top panel of Figure 2 summarizesthe ICME rate/solar (Carrington) rotation, aver-aged over 3 running rotations. The monthly sun-spot number is plotted in the second panel. Al-though the ICME rate broadly follows solar activ-ity levels, there is not a close correlation withsunspot number. Rather, the ICME rate maintainsa general level of ~2/rotation during much of theperiod with occasional intervals of enhanced ratesassociated with periods of exceptionally high

solar activity (such as July, 2000, early-2001).The ICME rate shows some evidence of quasi-periodic variations that are consistent with the~150 day periodicity previously reported invarious energetic solar data sets [3, 5]. The ICMErate has remained relatively high during the latedeclining phase of the solar cycle. Nevertheless,observations of low ICME rates during much of

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30TH INTERNATIONAL COSMIC RAY CONFERENCE 2007

2006, and during 2007 to date suggest that therate is approaching that seen in 1996 during thelast solar minimum.

Figure 3: Ulysses solar wind magnetic field,

plasma, and composition data showing twoICMEs in high-latitude, high-speed solar windflows. Expected values of Tp and comosi-tional/charge state parameters are overlaid in red.

We have previously noted [6], based on observa-tions during this and the previous two solar cy-cles, evidence of a solar-cycle dependence in thefraction of ICMEs that have the characteristics ofmagnetic clouds, i.e., an enhanced magneticfield strength with a smooth, slow rotation in fielddirection and low proton temperature. A MC

fraction of 30% is frequently quoted. However,we suggest that the fraction is dependent on thephase of the solar cycle, with the fewer ICMEsaround solar minimum having a higher incidenceof MCs than around solar minimum (third panelof Figure 2). The observations up to 2006 showonly a hint of the increase in the MC fractionexpected with a return towards solar minimumconditions.

Another ICME parameter showing an interestingvariation during this cycle is the in-situ ICMEspeed (Figure 2, panel 4). Average speeds have

increased by around 100 km/s, from ~400 to500 km/s. We also show speeds of individualICMEs. At least two factors may contribute tothis solar cycle variation. One is the increase inaverage solar wind speeds, also shown in thefigure (from the OMNI2 data set), since ICMEspeeds tend to converge towards the ambient solarwind speed. Another factor is the tendency forfast ICMEs (and fast CMEs at the Sun) to occurfollowing solar maximum in this cycle. An un-usual number of exceptionally fast shocks/ICMEshave been observed in this cycle, even during thelate declining phase, compared to recent cycles

(some fast events are absent in Figure 2 becauseof plasma instrument issues). The prevalence ofexceptionally fast transient events and associatedstrong shocks may help to account for the pre-dominance of solar energetic particle eventsdominated by shock acceleration during the sec-ond half of cycle 23 compared with the first half[7]. The shock/ICME in Figure 1 produced amodest geomagnetic storm of Dst = -47 nT.Around 90% of the ~100 storms in 1996-2005with Dst


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Figure 4: Ratio of the ICME rates at Ulysses andEarth during 1996-2001 as a function of time andUlysses heliolatitude.

Figure 3 shows two ICMEs observed by Ulyssesat 2 AU, 80N in high-latitude high-speed flows.

The ICMEs are readily identified by their anoma-lies with respect to expected values overlaid inred, including Tp

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Interplanetary Coronal Mass Ejections During 1996-2007