Abstract
The last solar minimum activity period, and the consequent minimum modulation conditions for cosmic rays, was unusual. The highest levels of galactic protons were recorded at Earth in late 2009 in contrast to expectations. Proton spectra observed for 2006 to 2009 from the PAMELA cosmic ray detector on-board the Resurs-DK1 satellite are presented together with the solutions of a comprehensive numerical model for the solar modulation of cosmic rays. The model is used to determine what mechanisms were mainly responsible for the modulation of protons during this period, and why the observed spectrum for 2009 was the highest ever recorded. From mid-2006 until December 2009 we find that the spectra became significantly softer because increasingly more low energy protons had reached Earth. To simulate this effect, the rigidity dependence of the diffusion coefficients had to decrease significantly below ∼ 3 GeV. The modulation minimum period of 2009 can thus be described as relatively more ‘diffusion dominated’ than previous solar minima. However, we illustrate that drifts still had played a significant role but that the observable modulation effects were not as well correlated with the waviness of the heliospheric current sheet as before. Protons still experienced global gradient and curvature drifts as the heliospheric magnetic field had decreased significantly until the end of 2009, in contrast to the moderate decreases observed during previous minimum periods. We conclude that all modulation processes contributed to the observed increases in the proton spectra for this period, exhibiting an intriguing interplay of these major mechanisms.
Similar content being viewed by others
References
Adriani, O., Barbarino, G.C., Bazilevskaya, G.A., Bellotti, R., Boezio, M., Bogomolov, E.A., et al.: 2009a, Phys. Rev. Lett. 102, 051101.
Adriani, O., Barbarino, G.C., Bazilevskaya, G.A., Bellotti, R., Boezio, M., Bogomolov, E.A., et al.: 2009b, Nature 458, 607.
Adriani, O., Barbarino, G.C., Bazilevskaya, G.A., Bellotti, R., Boezio, M., Bogomolov, E.A., et al.: 2011, Science 332, 69.
Adriani, O., Barbarino, G.C., Bazilevskaya, G.A., Bellotti, R., Boezio, M., Bogomolov, E.A., et al.: 2013, Astrophys. J. 765, 91.
Ahluwalia, H.S., Ygbuhay, R.C.: 2011, Adv. Space Res. 48, 61.
Aslam, O.P.M., Badruddin: 2012, Solar Phys. 279, 269.
Boezio, M., Pearce, M., Picozza, P., Sparvoli, R., Spillantini, P., Adriani, O., et al.: 2009, New J. Phys. 11, 105023.
Burlaga, L.F., Goldstein, M.L., McDonald, F.B., Lazarus, A.J.: 1985, J. Geophys. Res. 90, 12027.
Cliver, E.W., Richardson, I.G., Ling, A.G.: 2011, Space Sci. Rev. doi: 10.1007/s11214-011-9746-3 .
De Simone, N., Di Felice, V., Gieseler, J., Boezio, M., Casolino, M., Picozza, P., et al.: 2011, Astrophys. Space Sci. Trans. 7, 425.
Ferreira, S.E.S., Potgieter, M.S.: 2002, J. Geophys. Res. 107, SHPX:1-10.
Ferreira, S.E.S., Potgieter, M.S., Burger, R.A., Heber, B., Fichtner, H.: 2001, J. Geophys. Res. 106, 24979.
Ferreira, S.E.S., Potgieter, M.S., Moeketsi, D.M., Heber, B., Fichtner, H.: 2003, Astrophys. J. 594, 552.
Heber, B., Potgieter, M.S.: 2006, Space Sci. Rev. 127, 117.
Heber, B., Potgieter, M.S.: 2008, In: Balogh, A., Lanzerotti, L.J., Suess, T. (eds.) The Heliosphere Through the Solar Activity Cycle, Praxis, Chichester.
Heber, B., Kopp, A., Gieseler, J., Müller-Mellin, R., Fichtner, H., Scherer, K., et al.: 2009, Astrophys. J. 699, 1956.
Hoeksema, J.T.: 1991, In: Marsch, E., Schwenn, R. (eds.) Solar Wind Seven Colloquium, 191.
Jokipii, J.R., Kopriva, D.A.: 1979, Astrophys. J. 234, 384.
Kane, R.P.: 2011, Solar Phys. 269, 451.
Langner, U.W., Potgieter, M.S.: 2004, J. Geophys. Res. 109, 1.
Langner, U.W., Potgieter, M.S., Webber, W.R.: 2004, Adv. Space Res. 34, 138.
Langner, U.W., Potgieter, M.S., Fichtner, H., Borrmann, T.: 2006, Astrophys. J. 640, 1119.
McDonald, F.B., Webber, W.R., Reames, D.V.: 2010, Geophys. Res. Lett. 37, L18101:1-5.
Mewaldt, R.A., Davis, A.J., Lave, K.A., Leske, R.A., Stone, E.C., Wiedenbeck, M.E., et al.: 2010, Astrophys. J. Lett. 273, L1.
Mewaldt, R.A.: 2012, Space Sci. Rev. doi: 10.1007/s11214-012-9922-0 .
Moeketsi, D.M., Potgieter, M.S., Ferreira, S.E.S., Heber, B., Fichtner, H., Henize, V.K.: 2005, Adv. Space Res. 35, 597.
Ngobeni, M.D., Potgieter, M.S.: 2008, Adv. Space Res. 41, 373.
Ngobeni, M.D., Potgieter, M.S.: 2010, Adv. Space Res. 46, 391.
Ngobeni, M.D., Potgieter, M.S.: 2011, Adv. Space Res. 48, 300.
Nkosi, S., Potgieter, M.S., Ferreira, S.E.S.: 2008, Planet. Space Sci. 56, 501.
Nkosi, G.S., Potgieter, M.S., Webber, W.R.: 2011, Adv. Space Res. 48, 1480.
Parker, E.N.: 1965, Planet. Space Sci. 13, 9.
Potgieter, M.S.: 2000, J. Geophys. Res. 105, 18295.
Potgieter, M.S.: 2011, Space Sci. Rev. doi: 10.1007/s11214-011-9750-7 .
Potgieter, M.S., Ferreira, S.E.S.: 2001, Adv. Space Res. 27, 481.
Potgieter, M.S., le Roux, J.A., Burger, R.A.: 1989, J. Geophys. Res. 94, 2323.
Potgieter, M.S., le Roux, J.A.: 1992, Astrophys. J. 386, 336.
Potgieter, M.S., Moraal, H.: 1985, Astrophys. J. 294, 425.
Reinecke, J.P.L., Potgieter, M.S.: 1994, J. Geophys. Res. 99, 14761.
Scherer, K., Fichtner, H., Strauss, R.D., Ferreira, S.E.S., Potgieter, M.S., Fahr, H.-J.: 2011, Astrophys. J. 735, 128.
Strauss, R.D., Potgieter, M.S., Ferreira, S.E.S.: 2011, Adv. Space Res. 48, 65.
Strauss, R.D., Potgieter, M.S., Kopp, A., Büsching, I.: 2011, J. Geophys. Res. 116, A12105.
Shalchi, A.: 2009, Astrophys. Space Sci. Lib. 362, Springer, Berlin.
Teufel, A., Schlickeiser, R.: 2002, Astron. Astrophys. 393, 703.
Webber, W.R., Potgieter, M.S., Burger, R.A.: 1990, Astrophys. J. 349, 634.
Acknowledgements
The South African authors express their gratitude for the partial funding granted by the South African National Research Foundation (NRF) under the Italy–South Africa Research Cooperation Programme. They also thank the INFN for the partial financial support when visiting in Trieste and Rome over a span of three years.
Author information
Authors and Affiliations
Corresponding author
Rights and permissions
About this article
Cite this article
Potgieter, M.S., Vos, E.E., Boezio, M. et al. Modulation of Galactic Protons in the Heliosphere During the Unusual Solar Minimum of 2006 to 2009. Sol Phys 289, 391–406 (2014). https://doi.org/10.1007/s11207-013-0324-6
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s11207-013-0324-6