Magnetic Fluctuations and Peculiar Anisotropy Oscillations in the 23 July 2012 SEP Event Rick Leske, A. C. Cummings, C. M. S. Cohen, R. A. Mewaldt, A. W. Labrador, and E. C. Stone California Institute of Technology, Pasadena, CA USA M. E. Wiedenbeck Jet Propulsion Laboratory, Caltech, Pasadena, CA USA E. R. Christian and T. T. von Rosenvinge NASA/Goddard Space Flight Center, Greenbelt, MD USA 35th International Cosmic Ray Conference Busan, Korea 15 July 2017

The 23 July 2012 event: An ~X-class flare from W133° was associated with a large proton event and very fast (>2000 km/s) blast wave/shock (B>100 nT) at STEREO-Ahead. The SEP event was ~central meridian at Ahead, over the east limb from Behind, and ~43° over the west limb as seen from ACE.

Pitch Angle Distributions in the 23 July 2012 Event @ Ahead: At the event onset, 1-hour average pitch angle distributions are exponential beams. Time-intensity profiles in individual μ bins at 1-minute resolution show smooth rises in field-aligned directions, but large “oscillations” in μ bins that are not field-aligned: Leske et al., Proc. 34th ICRC (The Hague), paper 072 (2015); Leske et al., SW14, AIP Conf . Proc. 1720, 070004 (2016)

Later in the event, the fluctuations in beam width are much less. The width of the beam fluctuates in time, by >25° in a matter of minutes, and the peaks in pitch-angle distributions appear flattened. Later in the event, the fluctuations in beam width are much less. Leske et al., Proc. 34th ICRC (The Hague), paper 072 (2015); Leske et al., SW14, AIP Conf . Proc. 1720, 070004 (2016)

Changes in the character of the “oscillations” are well-correlated with changes in the variability of the magnetic field (longitudinal) directions:

The observed behavior can result if pitch angle distributions are calculated relative to field directions not aligned with the particle distribution symmetry axis (if calculated assuming gyrotropy). or Assuming gyrotropy

How could a mismatch between the magnetic field and energetic particle directions arise? Time offset between MAG and LET data? This was the case (see paper for details), but has been corrected here. “Oscillations” still present; we can rule out a timing offset as their primary cause. 2) Larger spatial region sampled by energetic particles than by SW in same 1-minute period: 6-10 MeV  <E>=7.75 MeV, <v>=0.13c; Vsw=430 km/s  LET energetic protons are 88x faster than the solar wind 7.75 MeV H gyroperiod in 4 nT field: ~ 17 sec. It takes SW ~25 min to cover the same distance these (field-aligned) protons do in a single gyroperiod!! …and the measured field direction fluctuates a lot during 25 minutes. Test by using longer time averaged (less rapidly varying) B field directions.

1) There was an 88-second time shift between MAG and LET data in our earlier reports. But correcting this offset, as shown here, produces no significant change in the character of the “oscillations” or the pitch angle distributions: A simple offset in the time tags is NOT the primary cause of the observed behavior. Same times 88-sec time shift (ICRC2015)

1) There was an 88-second time shift between MAG and LET data in our earlier reports. But correcting this offset, as shown here, produces no significant change in the character of the “oscillations” or the pitch angle distributions: Same times 88-sec time shift (ICRC2015)

2) But calculating pitch angle distributions using field directions averaged over (up to) 25 minutes makes a dramatic difference: These results indicate that the 1-minute angular distributions of the particles are much less variable than the 1-minute local field directions. Same times Up to 25-minute field averages

2) But calculating pitch angle distributions using field directions averaged over (up to) 25 minutes makes a dramatic difference: Same times Up to 25-minute field averages

Functional form used in fit: I = A*exp(B*cos(φ-φ0))+C Differences in field and particle directions can be more directly seen by fitting the (longitudinal) peak directions: Intensity Green = weighted fit to sectored intensities Blue = unweighted fit to sectored intensities Red= Magnetic field directions over 5 minute period Functional form used in fit: I = A*exp(B*cos(φ-φ0))+C Note: for small values of φ-φ0 , this is a Gaussian (plus offset), but cyclical in angle

Directions of the peak particle angular distributions (green and blue) obtained from the fits do not always agree with those of the local magnetic field direction (red)  distributions NOT gyrotropic about local field Also note: differences in field and particle directions are NOT simply due to a time offset.

Fluctuations in particle angular distributions relative to magnetic field directions were reported 30 years ago and were used to probe the turbulent structure of the magnetic field transverse to the solar wind flow direction (Bieber & Evenson, 1987 ICRC 3, 151). Would be interesting to perform a similar analysis with the 23 July 2012 data. Bieber & Evenson, 1987 ICRC 3, 151

Conclusions The apparent “oscillations” in pitch angle distributions during the 23 July 2012 SEP event are associated with relatively rapid variations in magnetic field direction and are the result of magnetic turbulence on the calculated particle pitch angle distributions. Fields governing energetic particle distributions are averages over larger spatial and temporal scales than those represented by solar wind in the same time interval. the directions of peak particle intensity and local, instantaneous magnetic field directions are not the same  the particle distributions are not gyrotropic relative to the local field. The standard method of calculating pitch angle distributions yields odd, misleading results for high energy particles when the field is varying rapidly.

STEREO/LET measures sectored rates in 16 viewing directions, arranged in 2 fans each covering 133° of longitude in the ecliptic and ±15-20° of latitude out of the ecliptic. Proton sectored rates (since 22 Nov 2010) are available at energies of 1.8-3.6, 4-6, and 6-10 MeV, and He at 4-6 and 6-12 MeV/n. Pulse height (PHA) data provide finer angular resolution in 300 (overlapping) viewing directions but at greatly reduced statistical accuracy. NOT VIEWED NOT VIEWED Mewaldt et al., SSR 136, 285 (2008)