 |
| Titel |
Open solar flux estimates from near-Earth measurements of the interplanetary magnetic field: comparison of the first two perihelion passes of the Ulysses spacecraft |
| VerfasserIn |
M. Lockwood, R. B. Forsyth, A. Balogh, D. J. McComas |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
0992-7689
|
| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 22, no. 4 ; Nr. 22, no. 4 (2004-04-02), S.1395-1405 |
| Datensatznummer |
250014846
|
| Publikation (Nr.) |
 copernicus.org/angeo-22-1395-2004.pdf |
|
|
|
|
|
| Zusammenfassung |
Results from all phases of the orbits of the Ulysses spacecraft have shown that
the magnitude of the radial component of the heliospheric field is approximately independent of
heliographic latitude. This result allows the use of near-Earth observations to compute the total
open flux of the Sun. For example, using satellite observations of the interplanetary magnetic
field, the average open solar flux was shown to have risen by 29% between 1963 and 1987 and
using the aa geomagnetic index it was found to have doubled during the 20th century. It is
therefore important to assess fully the accuracy of the result and to check that it applies to all
phases of the solar cycle. The first perihelion pass of the Ulysses spacecraft was close to sunspot
minimum, and recent data from the second perihelion pass show that the result also holds at solar
maximum. The high level of correlation between the open flux derived from the various
methods strongly supports the Ulysses discovery that the radial field component is independent
of latitude. We show here that the errors introduced into open solar flux estimates by assuming
that the heliospheric field's radial component is independent of latitude are similar for the two
passes and are of order 25% for daily values, falling to 5% for averaging timescales of 27 days or
greater. We compare here the results of four methods for estimating the open solar flux with
results from the first and second perehelion passes by Ulysses. We find that the errors are
lowest (1–5% for averages over the entire perehelion passes lasting near 320 days), for near-Earth
methods, based on either interplanetary magnetic field observations or the aa geomagnetic
activity index. The corresponding errors for the Solanki et al. (2000) model are of the order of 9–15%
and for the PFSS method, based on solar magnetograms, are of the order of 13–47%. The model of
Solanki et al. is based on the continuity equation of open flux, and uses the sunspot number to
quantify the rate of open flux emergence. It predicts that the average open solar flux has been
decreasing since 1987, as is observed in the variation of all the estimates of the open flux. This
decline combines with the solar cycle variation to produce an open flux during the second
(sunspot maximum) perihelion pass of Ulysses which is only slightly larger than that during the
first (sunspot minimum) perihelion pass.
Key words. Interplanetary physics (interplanetary magnetic
fields) – Solar physics, astrophysics and astronomy (magnetic
fields) |
| |
|
| Teil von |
|
|
|
|
|
|
|
|