Towards corpuscular diagnostics of coronal mass ejections at the Sun and in the interplanetary medium:

The 2 May 1998 event

Large solar energetic particle (SEP) events in the high-energy range, from tens MeV/n through a few GeV/n, occur in association with fast coronal mass ejections (CMEs) and flares and often start shortly after the flare (e.g., 9 July 1996). Hence, the SEP fluxes observed at 1 AU can carry information on the CME liftoff processes at/near the Sun.

Besides, solar eruptions often come in a series, so that SEPs of a new CME may sample the previous CME ejecta when traveling from near the Sun to 1 AU. One of such events was the 2-3 May 1998 SEP event observed onboard the Solar and Heliospheric Observatory (SOHO) spacecraft with the particle telescope ERNE/HED of the University of Turku.

Turku, 14 August 2007

Based on three papers in ApJ and one paper in JGR

by Torsti et al. (2004) and Kocharov et al. (2005, 2007)

EIT wave and dimming

SEP event of 2-3 May 1998

Particle and magnetic field measurements from Wind for the period 1800 UT, 1 May to 1200 UT, 4 May 1998 (Farrugia et al. 2002).

From top to bottom: the proton density, bulk speed, latitude, and azimuthal flow angles, proton (solid trace) and

electron (dashed trace) temperatures,

the GSM latitude and azimuthal direction of the

magnetic field, the total field, the solar wind dynamic

pressure, the proton beta, and the Alfvén Mach number. The dot-dash curve in panel 5

gives the expected temperature for normal solar wind expansion, after Lopez [1987].

IN SITU MAGNETIC CLOUD

The ejecta signatures comprise low kinetic temperature of ions, anisotropy of proton temperature, high helium abundance, strong magnetic field, rotation of magnetic field, and unusual ionization states of heavy ions (Neugebauer and Goldstein, 1997, and references therein).

Interplanetary CMEs can produce local depressions of the galactic cosmic ray intensity and are closely associated with bidirectional flows of galactic cosmic rays measured by the worldwide neutron monitor network (Richardson et al., 2000). Depressions typically extend over the ejecta regions as determined from a range of the ejecta plasma signatures.

Bidirectional flows of different particle populations were observed in regions of some ejecta. In particular there were frequent observations of bidirectional heat fluxes of solar wind electrons, ~100--200 eV (Goling et al., 1997). Counterstreaming of solar wind electrons indicate that magnetic field lines within ICMEs are connected to the Sun at both ends. However, the bidirectional electron flows are frequently observed to be intermittent (Shodhan et al., 2000). It has been suggested that this occurs when field lines inside the ICME are connected with open field lines of the normal solar wind. Occasionally the heat-flux is entirely absent, indicating a completely open structure. These mean that there are open field lines within some ejecta along which galactic cosmic rays may gain easy access and solar energetic particles may escape.

Dynamics of 17-22 MeV proton flux anisotropy observed on 2 May 1998

Modeling the SEP transport in interplanetary magnetic cloud

Equivalent model of the magnetic cloud for modeling of SEP transport

M. Vandas, D. Odstrčil, and S. Watari 2002, Three-dimensional MHD simulation of a loop-like magnetic cloud in the solar wind.

Steady-state (time-integrated) kinetic equation with no adiabatic deceleration included:

where the pitch angle shall be small: «1.

SEP transport along interplanetary magnetic field under weak scattering conditions

Adiabatic deceleration

Electromagnetic signature time (ES time)

Initial phase of the 2 May 1998 SEP event

(a): The 17-22 proton profile (open circles) is shown as observed with SoHO/ERNE for the protons registered at pitch-angle cosines ranging from 0.87 to 0.94. The observed profile of 26-40 MeV protons (crosses) is shifted by +11 min, to compensate for a velocity dispersion between the channels, re-normalized to the peak intensity of the 17-22 MeV channel, and shown only for the time >80 min. Dotted and solid curves show the Monte Carlo modeling results for the 17-22 MeV protons at two slightly different injection scenarios, 1 and 2, that are shown with corresponding curves in the lower panel.

(b): The solar injection rate profiles, 1 and 2, as functions of the ES time (the solar time with 8.3 min added). The vertical dashed line indicates the radio flash time and the estimated start time of the Moreton wave. The left, black histogram additionally shows the ERNE view-cone-integrated 1 min intensity of the 17-22 MeV protons, as a function of the ES time, i.e., the data are shifted by 39 min, then re-normalized to fit the exponential rise of the source function 2, to show when the production rate exceeded the actual background level. The right, blue histograms show the Goose Bay neutron monitor count rate shifted by 3 min, at two different re-normalization factors, with background subtracted. The red curve is for the re-normalized view-cone-integrated ERNE count rate in the 111-140 MeV proton channel, shifted by 11 min, with background subtracted.

(a, b) Re-normalized profiles of the ERNE view-cone-average proton intensity in different energy channels versus the time tES The broken exponential profile is the deka-MeV source profile 2 of previous figure.

(c) Corresponding re-normalization factors (solid histogram) and (d) Helium to proton abundance ratio.

The energy channel list with a color coding for panels (a-c) is given in panel (c).

Energy spectrum and composition ES-time profiles in 6 energy channels

Intermittency of the proton flux anisotropyIntensity-time profiles of 17-22 MeV protons arriving at different angles in respect to the magnetic field, from the southern directions (top panel; positive pitch-angle cosines) and from the northern directions (bottom panel). Nominal values of the pitch-angle cosines are shown in the figure legends. Points show the SOHO/ERNE data.

Periods of very high anisotropy (A, B, D, E) can be recognized by a strong separation of the series. They alternate with periods of moderate anisotropy (C, F).

Curves are for modeling that was optimized for the period B, with perfect trapping and the injection amplitudes QS=0.6 and QN=0.06 in the south leg and the north leg, respectively.10 AU (Case 1).

The ICME width is =25o

It is seen that a single-loop model cannot explain all the periods of the event. In particular, at the period D onward the model predicts much more counter-streaming particles than observed (bottom panel).

A fit for time-angle profiles of 17-22 MeV protons

Intensity-time profiles of protons arriving at different angles in respect to the magnetic field, from the southern directions (top panel) and from the northern directions (bottom panel). Nominal values of the pitch-angle cosines are shown in the figure legends. Points show the SOHO/ERNE data.

The modeling curves comprise Case 2A (with injection amplitudes QS=QN=0.7, Case 2B (QS=0.6, QN=0.1), Case 2C (QS=2.3, Q N=11), and Case 3D (QS=12, QN=0.7) for the periods A, B, C, and D, respectively.

Model parameters

Pitch angle distributions of 17-22 MeV protons in different periods of the 2 May 1998 event

Proton flux anisotropy and magnetic fluctuations

Intermittent behavior of the 17-22 MeV proton flux anisotropy (histogram) and the energy density of selected magnetic fluctuations at 1~AU (curve). The anisotropy is measured as a reciprocal of the natural logarithm of the ratio of proton intensities at two directions, =0.8$ and =0.5. The selected magnetic fluctuations are of a spatial scale comparable to the proton Larmor radius

Conclusions: Flare and CME at the Sun

1. In the deka-MeV energy range (~10-50 MeV) we observe a prompt and energy-dependent rise of the particle production at/near the Sun, with the first 20 MeV proton release in not more than 4 min after the radio flash and the estimated Moreton wave start. A doubling of the proton energy took ~5 min, as estimated from the energy-dependent delay of the proton production rise.

2. The history of hecto-MeV proton onset was significantly different. Their rise was delayed in respect to the 20 MeV proton rise by 10-15 min and was independent of energy inside the hecto-MeV band.

3. The first phase of the deka-MeV proton production was associated with EIT wave, whereas the hecto-MeV proton release was associated with EIT dimming.

4. The first, energy-dependent rise of deka-MeV proton production was followed by the second phase increase, which was energy-independent. Then production of all deka-MeV protons very slowly decayed, with the e-fold time of 4 hours.

A straightforward explanation for these findings is a coronal, CME-liftoff associated acceleration in a variable magnetic environment, followed by a gradual reacceleration. The hecto-MeV protons may be initially accelerated on closed magnetic field lines and then released after the transient coronal hole formation.

Conclusions: Interplanetary CME

1. The analysis has revealed a strong intermittency of the SEP transport parameters, when different magnetic tubes were convected past the spacecraft. The estimated mean free path values vary over one order of magnitude, from ~2 AU to ~20 AU.

2. The SEP event can be modeled with a prolonged injection of particles from a new CME into the previous ejecta comprising a set of magnetic loops. Angular spread of a loop met in the beginning of the SEP event was ~25º.

3. A lack of bounced protons observed later in the event can be explained either by a fast escape of protons from the narrow, ~25º loop, or by a proton injection predominantly into one leg of a wide, >60º loop.

4. An imprint of the magnetic compression at the leading part of CME is found in the proton flux anisotropy observed in the beginning of the event.

These findings illustrate how high-precision anisotropy measurements and a numerical modeling can be used as a test of the ICME structures. Future 3D modeling and more comprehensive observational data can help us to resolve ambiguities of the data interpretation and reduce uncertainties in the parameter estimates.