WHERE WERE THE ANOMALOUS COSMIC RAYS WHEN VOYAGER 1

CROSSED THE TERMINATION SHOCK?

Frank B. McDonald

Institute for Physical Science and Technology, Univ. of Maryland, College Park, MD

University of MarylandSpace and Cosmic Ray Physics Seminar

30 October 2006

• Heliosheath at the nose is estimated to be 30-60 AU wide.

• At the TS, radial velocity of the solar wind will decrease by a factor of 2.4 – 4 depending on the strength of the TS, and will continue to decrease as 1/r2 where r is the heliocentric distance.

• The intensity of the transverse component of the interplanetary magnetic field jumps by the same factor and continues to increase proportional to r across the heliosheath.

HELIOSHEATH

a

MAGNETIC FIELD LECP

WHAT WAS EXPECTED AT THE TERMINATION SHOCK

• 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 GCR ion and electron intensity were expected as Voyager 1 approached the TS.

• The termination shock is the most probable source of ACRs.

Would expect to observe the ACR source spectra.

• 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.

WHAT WE SAW

• V1 crossed the heliospheric TS on Dec. 16, 2004 at 94 AU and entered the region of the heliosheath, where it has remained for almost 2 years.

• At the TS the ACR intensity > 4 MeV/n was well below the predicted level and significantly below that observed for the first V1 TS Particle event starting in 2002.54 at 85 AU.

The modulation state of the heliosphere:

• In late 2004, the GCR recovery at 1 AU from solar maximum to solar minimum conditions was 75% complete. This recovery is also ongoing at the Voyagers.

• From 2002.54 to the TS crossing V1 has been observing TSP events for ~80% of the time as it traveled some 9 AU. These TSP events provide a reasonable measure of the ACRs at mid-heliolatitudes.

WHAT WE THINK IS GOING ON

After 22 months of V1 observations in the heliosheath and the simultaneous V2 observations of TSPSs, we conclude:

At energies > ~4 MeV/n, TSP events are an effective monitor of ACRs at mid heliolatitudes at the TS, (ie TSPs = ACR)

The data suggest 3 principal effects to explain the low ACR intensity at the TS on 16 Dec 2004:

• The large interplanetary transients associated with the intense Oct/Nov 2003 “Halloween” solar events and subsequent solar activity play a major role in reducing the energetic particles over the ~4 month period prior to the TS crossing.

• There is a long term variation of the ACRs that appears to track the 11 year galactic cosmic ray variation.

• The intensity of low energy ACRs (ie He < ~15 MeV) is significantly affected by the polarity reversal of the interplanetary magnetic field near the time of solar maximum.

The V1/V2 ACR Variations Are Primarily Temporal And Not Spatial

Spacecraft Mass: 825 kgPower: RTG (Plutonium Oxide)Antenna: 3.7 mRequires 34 m dishBit Rate: 160 bits/s

VOYAGER SPACECRAFT

Science TeamE.C. Stone (PI), A.C. Cummings (Caltech)N. Lal, B.C. Heikkila (GSFC)F.B. McDonald (Univ. of Maryland)W.R. Webber (New Mexico State Univ.)

Energetic Particle Coverage H: 1.8-300 MeV He: 1.8-650 MeV/n Z = 1-28 (Resolves Isotopes) E: 2.5 – 140 MeV

CRS EXPERIMENT

ANOMALOUS COSMIC RAYS

Effect Of Strong Transients At V1 Just Prior To Crossing The TS

Comparison of V1/V2 in the Foreshock Region and V1 in the Heliosheath

Relation Of The V2 TSP Increase And V1 ACRs In The Heliosheath

Reasonable Correlation Between V2 in the Foreshock Region and V1 in the HeliosheathTSP ACRs

Intensity Variations at this Point are Predominantly Temporal and not Spatial

Relation Of The V2 TSP Increase And V1 ACRs In The Heliosheath

V1 ACR 10-21 MeV/n He versus 150-380 MeV/n He

V2 ACR 10-21 MeV/n He versus 150-380 MeV/n He

22 YEAR HELIOMAGNETIC CYCLE

DISCUSION

• The V1/2 ACR and GCR observations over solar cycle 23 suggest there is both an 11 year and 22 year variation in the intensity of ACRs at the TS based on the close correlation between these two energetic particle populations over this period.

• Different physical processes in the ACR and GCR long term temporal variations with each process controlled by phenomena associated with some aspect of the long term activity cycle such as GMIRS, current sheet inclination, changes in the interplanetary diffusion coefficients, ect.

• However, the higher energy ACRs require acceleration times on the order of months. The close correspondence between both the long term and short term ACR and GCR temporal variations indicate there could be common changes in their transport and acceleration processes.

• The strong effect of GMIRs on ACRs at V1 over the period 2002.54 - 2004.93 indicate they are a major factor.

(a) (b) (c) (d) (e) (f)

Time TS Location

V2 V(V2) V

(V2 Sun)

Time at Sun

Voyager 1

( = 34N)

Onset of Reversal

2001.31 88 AU 2000.9

59.48 AU

370 km/s 410 km/s 2000.2

mid March

Completion of Reversal

2001.61 88 AU 2001.25

60.53 AU

375 km/s 408.6 km/s 2000.54

mid July

Voyager 2

( = 26S)

Onset of Reversal

2001.39 86 AU 2001.12

63.2 AU

412 km/s 447 km/s 2000.45

mid June

Completion of Reversal

2001.81 86 AU 2001.52

64.47 AU

366 km/s 401 km/s 2000.76

early October

TABLE 1

Heliospheric Termination

Shock

Interplanetary Magnetic Field Photosperic Magnetic Field

Voyager Observation

Wang et. al. Durant & Wilson (2003)

Harvey &

Recely

(2002)

Bilenico (2002)

Gopalswarmy

(2003)

Northern Hemisphere

Onset of Reversal

2000.2

(mid March)

January

2000

6 May

2001

March

2001

February 2001

November 2000

Completion of Reversal

2000.54

(mid July)

April

2000

2 June

2001

Southern Hemisphere

Onset of Reversal

2000.45

(mid June)

June

2000

19 September 2001

May

2001

April 2001

May 2002

Completion of Reversal

2000.77

(early Oct.)

November 2000

17 October

2001

TABLE 2

2- 160 MeV ELECTRONS IN THE HELIOSHEATH

• Highly relativistic, low rigidity electrons (10 MeV = 10MV)

• Very sensitive to modulation conditions

• Still ~250 less than predicted IS intensity (at 10 MeV)

• 2-25 MeV Electrons have large response to IP transients in the distant heliosphere.

CONCLUSIONS

1. With time, spatial effects will become dominant as Voyager travels across the heliosheath and as we approach solar minimum conditions (Late 2007 at V1?)

2. After solar minimum we expect to see a decrease in the ACR intensity due to both increasing distance from the source region (the TS) and long term modulation effects at the TS.

3. Relative V1/V2 GCR intensities at solar minimum should provide some measure the magnitude of GCR acceleration at the TS. Based on cycle 23, significant solar cycle modulation of GCRs in the heliosheath is not expected.

4. GCR electrons will be especially valuable in monitoring transients in the heliosheath and in establishing the modulation effectiveness of the heliosheath.