Latitudinal structure and north-south asymmetry of the solar wind from Lyman-αremote sensing by SWAN

Abstract

Abstract. Based on SWAN/SOHO observations carried out during 1996-2002, we analyze latitudinal profiles of the heliospheric backscatter Lyman-α radiation. We use these results to investigate the ionization field of neutral hydrogen in the inner heliosphere and the latitudinal distribution of the solar wind mass flux. The the depth and latitudinal range of the equatorial depression in the Lyman-α backscatter glow (the so-called "groove") are correlated with the corresponding parameters of the ionization field. We show that the groove is entirely due to latitudinal anisotropy of the solar wind, since, as we are able to demonstrate, the photoionization rate remains spherically symmetric throughout the solar cycle. During the last solar minimum the groove was well developed and stable. During the ascending phase of solar activity, it expanded in latitude (first south, then north), and disappeared altogether during the solar maximum. Shortly after the maximum it reappeared, but its structure was more complex than during the ascending phase. The groove feature is correlated with the equatorial band occupied by the slow solar wind, while the polar maxima of the Lyman-α intensity correspond to the fast solar wind from the polar holes. The groove observations (supported by appropriate modeling) show that during the last solar minimum the mass flux of the fast solar wind from the north and south polar holes were different from each other: a true north-south asymmetry between the polar regions was detected. During the solar minimum, the area occupied by the slow solar wind was quite stable and offset slightly to the south with respect to the solar equator: it extended to about 30 • N and 35 • S from the beginning of observations in May 1996 till 1998. Then it expanded by about 10 • north and south, and subsequently migrated towards southern latitudes, so that it engulfed the south pole in May/June 2000. The north region of the fast wind survived longer and disappeared as late as November/December 2001. To check for the persistence of the north-south asymmetry, we analyze as a proxy the net sunspot area in the north and south hemispheres of the Sun during the 12 past solar cycles. Small north-south asymmetries are found to be commonplace during the past cycles, but the polarity of the asymmetries changes, leaving no statistically significant remnant asymmetry. This suggests that the solar dynamo is solely responsible for the asymmetry, with no remnant magnetic field from the protosolar nebula.

Survival probability and energy modification of hydrogen energetic neutral atoms on their way from the termination shock to Earth orbit

“…Kyrölä et al 1998;Bertaux et al 1999). Bzowski (2003) proposed a quantitative model to infer the latitudinal span of the equatorial band of the slow wind from the observations of the latitudinal range of the groove, employed by Bzowski et al (2003) to determine the evolution of the slow-wind region from solar minimum in 1996 to solar maximum in 2001. Bzowski et al (2008) pointed out that there is a correlation between the latitudinal span of coronal holes and the boundaries of the slow wind region.…”

3-D-geocoronal hydrogen density derived from TWINS Ly-α-data

“…Bzowski et al, 2003) and needs to be removed as an additional radiation contribution. Intensities of this interplanetary glow are strongly varying with ecliptic view directions and with the phase of the solar activity cycle (see Fahr, 2004, for a recent review).…”

Density of neutral interstellar hydrogen at the termination shock from Ulysses pickup ion observations

“…The groove is due to the latitudinal anisotropy of the solar wind, making it a good tracer for the latitudinal structure (Bzowski 2003). Bzowski et al (2003) exploited this to infer the equatorial-to-pole contrast of the charge exchange rates and the latitudinal boundaries of the polar regions of reduced rates for selected dates between solar minimum and maximum.…”