The Effect of the Heliosphere on Galactic and Anomalous Cosmic Rays

Principal Author: Israel, Martin H. Washington University in St. Louis [email protected] 314-935-6263

Co-Authors: Leske, Richard A. California Institute of Technology Binns, W. Robert Washington University in St. Louis Christian, Eric NASA-Goddard Space Flight Center Cummings, Alan C. California Institute of Technology Labrador, Allan W. California Institute of Technology Lave, Kelly A. Washington University in St. Louis de Nolfo, Georgia A. NASA-Goddard Space Flight Center von Rosenvinge, Tycho T. NASA-Goddard Space Flight Center Wiedenbeck, Mark E. Jet Propulsion Laboratory

Brief White Paper Description: The interplanetary magnetic field modulates both Galactic and anomalous cosmic rays in the inner solar system. GCR and ACR observed during the recent solar minimum show that the modulation is not well understood. Observations over at least the next 22 years are needed to resolve this puzzle.

This White Paper addresses the following relevant panels: Solar and Heliospheric Physics (SHP)

The interplanetary magnetic field (IMF) of the heliosphere has a major effect on the intensity of the Galactic cosmic rays (GCRs) in the inner solar system. The GCR intensity reaches a maximum during solar minimum, when the sunspot number and other indicators of solar activity are at a minimum and declines substantially when solar activity increases. This modulation of the GCR intensity is most pronounced at energies below a few GeV/nucleon, at energies near and below the peak in the GCR energy spectrum. Since the GCRs are a major component of the radiation environment to which astronauts and spacecraft are subjected, these variations in intensity have implications for both human and robotic space exploration.

Although this solar modulation of GCRs has been studied extensively over several solar cycles, the most recent (2008 – 2010) solar minimum has been surprisingly different from those previously studied. This minimum of solar activity has lasted longer than previous minima, and the intensity of low-energy GCRs in 2010 reached a level more than 20% higher than has ever been recorded since the beginning of the space age sixty years ago (Mewaldt, et al., 2010).

Figure 1 shows the intensity of GCR Oxygen over the past thirteen years. Also shown is the expected intensity predicted in 2007 extrapolated from the neutron monitor data to that date, based on the observations from previous solar minima. The difference between the projection and the actual data for the past three years demonstrates that the solar cycle and its effect on GCRs is still imperfectly understood. This figure also shows several parameters describing the interplanetary medium, each of which is likely to have some effect on the local GCR intensity. The relative effect of each of these parameters on the GCRs is still not well understood.

Since the sunspot number on the Sun reaches a maximum approximately every eleven years, the solar cycle is often thought of as being approximately eleven years in duration. In fact the cycle of solar modulation of GCRs is about twenty-two years. Every eleven years the polarity of the solar magnetic field reverses, and the reversed polarity, combined with the spiral shape of the interplanetary magnetic field, leads to a marked difference in GCR modulation between the two halves of this twenty-two year cycle. (Jokipii, Levy, & Hubbard., 1977; Jokipii & Thomas, 1981). If we are to untangle the various effects on local GCR intensity and understand the present “anomalous” solar minimum, it will be necessary to monitor the GCR intensity and the various interplanetary parameters that affect it for at least the next twenty-two years. In such studies, it is particularly useful to have long-term continuous data sets from the same instrument. The low-energy oxygen data shown in both figures of this white paper come from thirteen years of data from instruments on the ACE spacecraft, and we note that the consumables on ACE are adequate to continue the mission to ~2024. At higher energies long-term data sets from ground-level neutron monitors are also valuable.

A vital tool in developing an understanding of solar modulation of GCRs is comparison between measurements of GCRs near Earth at 1 AU with measurements in the outer heliosphere and beyond. When Voyager-1 crosses the heliopause there will be a once- in-a-lifetime opportunity to compare directly-measured GCR spectra in interstellar space

- 1 - with spectra in the outer heliosphere measured by Voyager-2 and with spectra at 1 AU, measured by instruments on spacecraft like the Advanced Composition Explorer (ACE). In addition to tying down the major unknown in the modulation problem – the interstellar spectrum – we will also finally know the maximum GCR intensity to which Earth can be subjected in times like the Maunder minimum. Only then will it be possible to decode accurately the 10Be cosmic-ray record in terrestrial samples.

In addition to GCRs, anomalous cosmic rays (ACRs) provide an important probe of solar modulation in the heliosphere. ACRs are produced when neutral atoms from the local interstellar medium drift into the heliosphere, where they are ionized and convected to the outer heliosphere where they are accelerated, possibly by diffusive shock acceleration at the termination shock. Since they are accelerated at the outermost parts of the heliosphere, one would expect that they would suffer the same modulation as they drift into 1 AU as do the GCRs.

As shown in Figure 2, although GCRs were at record-high intensities this solar minimum, ACRs at 1 AU were not (Leske et al., 2010). In fact they were quite typical, with peak intensities similar to those during the last solar minimum with A<0 polarity in the mid- 1980's and below those of the A>0 minimum of ~11 years ago. Voyager measurements in the outer heliosphere show that the intensity of high-energy ACRs was the same this solar minimum as it was in the mid-1980's, suggesting that the source intensity of ACRs was unaffected by the changes in the interplanetary parameters shown in Figure 1.

During A<0 cycles ACRs drift into the inner heliosphere along the heliospheric current sheet (HCS) (Jokipii and Thomas, 1981), and as Figure 2 indicates there was a striking correlation between the HCS tilt angle, ACR intensities, and GCR (neutron monitor) intensities during the 1980's A<0 epoch. Prior to 2000, the major, prolonged deviations between the tilt angle and cosmic ray intensities occur during the approach to solar maximum from the two A>0 minima, when the neutron monitor and ACR rates declined only slowly as the tilt angle rapidly increased. This is as expected: since particles drift in from the polar regions of the heliosphere during A>0 periods, they are virtually unaffected by changes in the largely near-equatorial HCS, at least until the HCS reaches high latitudes. However, after the last field reversal in ~2000, the neutron monitor rate, ACR intensity, and HCS tilt angle no longer scale as they did for the previous three solar minima. For a given tilt angle, both GCR and ACR intensities are elevated compared with the last A<0 solar minimum, but GCRs are relatively more enhanced than ACRs. Although the long, deep solar minimum allowed for less cosmic-ray modulation, the relatively high HCS tilt angle partially compensated for this. If the ACR source distribution is concentrated at low latitudes during A<0 periods (Jokipii and Giacalone, 1998), the ACRs may be more sensitive to the high tilt angles than the GCRs were.

Discovering the puzzle that solar modulation can yield record high GCR intensities without affecting the ACRs required observations in both the inner and outer heliosphere across a broad range of particle energies throughout more than one full 22-year solar magnetic cycle. Resolving the puzzle, and testing any proposed resolution, will require that such observations continue for future solar cycles, and will likely better elucidate the

- 2 - underlying physics of solar modulation, the origin of ACRs, and the interaction of our Sun and heliosphere with the local interstellar medium.

Observations required

As indicated above, an understanding of these heliospheric effects on the interplanetary radiation environment will require continuous observations of both the galactic and the anomalous cosmic rays over at least the next twenty-two-year solar cycle. Ground-level neutron monitors are helpful in this regard, but their mean response is to particles with energy ~10 GeV, while the bulk of the ionizing radiation is at lower energies. Thus particle detectors near 1 AU outside the Earth’s magnetosphere (like ACE at L1) or in low-earth polar orbit or in highly eccentric earth orbit (like IMP-8) will be needed. Detectors on the ACE spacecraft have been serving this function superbly for the past thirteen years, and that spacecraft has consumables capable of extending those observations until ~2024. Continuation of the ACE mission as long as the spacecraft and its cosmic-ray instruments last will be the least expensive way of assuring continuation of this capability. Beyond the ACE mission, a relatively simple cosmic-ray telescope with capability of identifying individual elements could be placed on a polar orbiting spacecraft to provide a low-cost means of extending this capability to twenty-two years from now, and beyond. Such continuous monitoring of the cosmic-ray intensity will be vital for planning of future human and robotic exploration of our Solar System.

- 3 - Figure 1 a) GCR oxygen intensity

b) Average tilt angle of the heliospheric current sheet

c) Mean strength of the interplanetary magnetic field.

d) Mean free path of particles parallel to the local magnetic field

e) Solar wind speed

f) Dynamic pressure of the solar wind

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Figure 2: Comparison of the ACR intensity (red data points; first left axis) and the Newark neutron monitor count rate (blue curve; second left axis) with the tilt angle of the heliospheric current sheet from the Wilcox Solar Observatory "classic" line-of-sight model (http://wso.stanford.edu; thick black curve; right axis -- note inverted scale) over the past several solar cycles, from Leske et al. (2010).

- 5 - References:

J.R. Jokipii, E.H. Levy, & W.B. Hubbard (1977) Effects of Particle Drift on Cosmic-ray Transport. I. General Properties, Application to Solar Modulation, Astrophys. J. 213, 861.

J.R. Jokipii & B. Thomas (1981) Effects of Drift on the Transport of Cosmic Rays IV. Modulation by a Wavy Interplanetary Current Sheet", Astrophys. J. 243, 1115.

J.R. Jokipii and J. Giacalone, (1998) The Theory of Anomalous Cosmic Rays, Space Sci. Rev. 83, 123.

R.A. Leske, A.C. Cummings, R.A. Mewaldt, and E.C. Stone (2010) Anomalous and Galactic Cosmic Rays at 1 AU During the Present Solar Minimum, Space Sci. Rev., submitted.

R.A. Mewaldt, A.J. Davis, K.A. Lave, R.A. Leske, E.C. Stone, M.E. Wiedenbeck, W.R. Binns, E.R. Christian, A.C. Cummings, G.A. deNolfo, M.H. Israel, A.W. Labrador, & T.T. vonRosenvinge, (2010) Record-Setting Cosmic-Ray Intensities in 2009 and 2010. Astrophys. J. Lett. 723, L1-L6.

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