Voyager Observations of Galactic and Anomalous Cosmic Rays in the Heliosheath

1. Voyager Observations of Galactic and Anomalous Cosmic Rays in the Heliosheath F.B. McDonald1, W.R. Webber2, E.C. Stone3, A.C. Cummings3, B.C. Heikkila4 and N. Lal4 1 Institute for Physical Science and Technology, University of Maryland, College Park, MD, USA 2 Department of Physics and Astronomy, New Mexico State University, Las Cruces, New Mexico, USA 3 California Institute of Technology, Pasadena, CA, USA 4 NASA/Goddard Space Flight Center, Greenbelt, MD, USA Ed Stone Symposium February 11, 2006

2. INTRODUCTION Voyager 1 crossed the heliospheric Termination Shock (TS) on December 16, 2004 at 94AU and entered the region of the heliosheath, where it has remained for more than 1 year. • At the termination shock the ACR intensity > 10 MeV/n was well below the predicted level (Cummings, Stone, Steenburg, 2002). • In fact the ACR He intensity > 10 MeV/n and that of 2-20 MeV electrons was significantly below that observed for the first Voyager 1 Termination Shock Particle (TSP) event starting in 2002.54 at 85AU. • From 2002.54 to the TS crossing Voyager 1 has been observing TSP events for ~80% of the time as it traveled some 9 AU.

3. At 2004.87, TSP, ACR and GCR intensities were at a low level at Voyager 1 due to the passage of a series of interplanetary transients. Beginning at this minimum (2004.87), some 30 days before the TS crossing, there began an increase at Voyager 1 of ACRs, GCRs, and 2-20 MeV electrons that has continued up to the present time. • These intensity increases are also present in the Voyager 2 data which suggests that the increases are dominated by temporal and not spatial effects. • The first Voyager 2 TSP event begins in ~2004.87, consistent with the onset of the Voyager recovery. It will be argued that the MIR produced by the October/November 2003 Halloween events and subsequent solar activity over the ensuing 90 days is the major cause of the low ACR and electron intensity at the TS crossing. This does not rule out additional factors such as the motion of the TS or more efficient acceleration at other locations along the TS such as the equatorial region.

4. WHAT WAS EXPECTED • The TS is the most probable source of the ACR’s. Would expect to observe the ACR source spectra. Pesses, Jokipii and Eichler, 1981 Cummings, Stone and Steenberg, 2002 • GCR ions and electrons traverse the heliosheath and cross the TS before interacting with the supersonic solar wind. GCR ions may experience modest local reacceleration through their encounter with the TS. This interaction could be a major effect for low energy galactic electrons (1-100 MeV). Increases in the GCR ion and electron intensity were expected as Voyager 1 approached the TS. Jokipii et al., 1992 Steenberg, PhD Thesis Potgieter and Ferreira, 2002 Caballero et al., 2004 • The TS may play a role in further accelerating the solar/interplanetary (S/IP) ions associated with the merged interaction regions that sweep across it.

5. THE MODULATION STATE OF THE HELIOSPHERE • In late 2004, the GCR recovery at 1 AU from solar maximum to solar minimum conditions was ~50% complete. This recovery is also ongoing at the Voyagers.

6. At 1AU the intensity of 150 - 380 MeV/n He from solar minimum to solar maximum of cycle 23 was reduced by a factor of ~5 while at V2 (64AU) the reduction was 30% and at V1 (82AU) the reduction was 22%. Near the ecliptic plane, the effects of the ll year solar modulation appear to be confined to the domain of the supersonic solar wind.

7. The Effects of Interplanetary Transients on the Voyager 1 Termination Shock Particle Events Prior to Crossing the TS From 2001.0 – 2004.5, Voyager 2 (some 19AU closer to the Sun than Voyager 1) observed a series of 9 increases of MeV ions that generally persisted over 3-4 solar rotations. • Accompanying increases in solar wind velocity V and interplanetary magnetic field B. • A decrease in the GCR rate (an 8th increase, Event 6, is contained between 2 MIRs). • The events occur with quasi-periodicity of ~150 days. • 7 of these events were associated with the passage of a merged interaction region marked by:

8. Before the onset of TSP 1 at Voyager 1 there is a close correspondence between the Voyager 1 and Voyager 2 time histories of MeV particles after an appropriate convection correction has been applied. • After the onset of TSP 1 at Voyager 1, the passage of interplanetary transients have a strong effect on the TSP time histories and flow patterns that appears to be greater for MeV electrons, higher energy H ions (> ~15 MeV) and possibly others. • TSP 1 and 2 start immediately after the passage of MIRs associated with Event 4 and 7. • TSP 1 is terminated by the passage of Event 6. • The MIR of Event 8 produces a sharp decrease in the Voyager 1 TSP 2 intensity. There is an enhancement of MeV electrons associated with the passage of MIRs 5 and 6 and following Events 4, 8 and 9. We believe these electrons are of galactic origin that have been reaccelerated at the TS with the IP transiet also playing a role.

9. A more detailed look at the effects of MIR 8 and 9 on MeV electrons, GCRs and both higher and lower energy H is shown below. The sharp decease in 2.5 MeV H produced by the passage of MIR 8 is followed by an increase in all 4 components (2004.1-2004.23) with the temporal structure of the TSPs being similar to that of the GCR. After the predicted time of passage of MIR 9 there are especially strong increases and fluctuations in the intensity of MeV electrons and the higher energy ions.

10. The enhancements of 43 MeV H following events 8 and 9 show significant streaming along the expected direction of the IP B field and there is a strong correspondence between the temporal variations of these streaming ions and 2.5 – 14 MeV relativistic electrons. These fluctuations over the period 2004.57 – 2004.87 are associated with the strong Halloween (2003) solar events and subsequent solar activity. The largest fluctuations are associated with enhanced magnetic B fields. These electrons and ions are most probably produced by the MIR/TS interaction. The electrons, 2.5 and 43 MeV H and GCRs all reach a minimum on 2004.87 and begin their recovery some 30 days before Voyager 1 crosses the TS. The first detection of Voyager 2 TSPs also occurred at this time.

11. Energetic Particle Observations in the Heliosheath The intensity of GCRs, ACRs, MeV electrons and TSPs at the TS crossing were at a lower level than that observed for TSP 1 and 2.The time histories of a series of Voyager 1 / Voyager 2 H and He intervals for the period 2004.5 – 2005.9 are shown below. We used 26-day averages to reduce short-term fluctuations. A similar plot for GCR H, He and 2.5-14 MeV electrons is also shown. All of these TSPs, ACRs and GCRs show somewhat similar rates of increases at Voyager 1 and Voyager 2 after 2004.87.

12. To compare these increases in a more direct way, the Voyager 1 and Voyager 2 data are normalized over a 26-day period that approximately straddles the TS crossing with no time shifting.Except for 2.5-14 MeV electrons and 30-56 MeV H, the relative intensity increases are not markedly different at the 2 spacecraft despite their locations at 34N and 26S. The Voyager 2 increases in H, He and electrons after 2004.87 mark the onset of the first TSP event at Voyager 2. At the present time the peak intensity of the Voyager 2 2.5 MeV ions is approaching that of the Voyager 1 TSP1 and is greater than any of the Voyager 2 S/IP events.

16. CONCLUSIONS • From 2002.54-2004.96 Voyager 1 has been monitoring energetic particles at the TS near heliolatitudes of 34N using long lasting TSP events. The reduced intensity levels observed at the actual TS crossing resembles previously observed minima following the passage of large IP transients. The transients associated with the intense Halloween events and subsequent solar activity play a role in reducing the energetic particles at the TS just prior to the TS crossing. • The recovery of GCRs at the Voyagers in late 2004 is closely related to the ongoing recovery at 1AU. • The energetic particle increases at the Voyagers after 2004.87 would appear to be predominantly temporal and not spatial – except for the 2.5-14 MeV electrons. The larger increase in 30-56 MeV H at Voyager 1 is not understood. • Other factors, such as movement of the TS or preferential acceleration in other regions of the TS may also be important.