Accession Number:

ADA561709

Title:

Solar Drivers of 11-yr and Long-Term Cosmic Ray Modulation (PostPrint)

Descriptive Note:

Technical paper 1 Oct 2007-24 Feb 2011

Corporate Author:

AIR FORCE RESEARCH LAB KIRTLAND AFB NM SPACE VEHICLES DIRECTORATE

Report Date:

2012-03-06

Pagination or Media Count:

20.0

Abstract:

In the current paradigm for the modulation of galactic cosmic rays GCRs, diffusion is taken to be the dominant process during solar maxima while drift dominates at minima. Observations during the recent solar minimum challenge the pre-eminence of drift at such times. In 2009, the 2 GV GCR intensity measured by the Newark neutron monitor increased by 5 relative to its maximum value two cycles earlier even though the average tilt angle in 2009 was slightly larger than that in 1986 20 vs. 14 , while solar wind B was significantly lower 3.9 nT vs. 5.4 nT. A decomposition of the solar wind into high-speed streams, slow solar wind, and coronal mass ejections CMEs including postshock flows reveals that the Sun transmits its message of changing magnetic field diffusion coefficient to the heliosphere primarily through CMEs at solar maximum and high-speed streams at solar minimum. Long-term reconstructions of solar wind B are in general agreement for the 1900-present interval and can be used to reliably estimate GCR intensity over this period. For earlier epochs, however, a recent 10Be-based reconstruction covering the past 104 years shows nine abrupt and relatively short-lived drops of B to 0 nT, with the first of these corresponding to the Sp rer minimum. Such dips are at variance with the recent suggestion that B has a minimum or floor value of 2.8 nT. A floor in solar wind B implies a ceiling in the GCR intensity a permanent modulation of the local interstellar spectrum at a given energyrigidity. The 30 40 increase in the intensity of 2.5 GV electrons observed by Ulysses during the recent solar minimum raises an interesting paradox that will need to be resolved.

Subject Categories:

  • Astrophysics

Distribution Statement:

APPROVED FOR PUBLIC RELEASE