Solar wind decrease at high heliographic latitudes detected from Prognoz interplanetary lyman alpha mapping

Abstract

New evidence for a latitudinal decrease of the solar wind mass flux is presented from observations of the interplanetary Lyman alpha emission collected in 1976 and 1977 with satellites Prognoz 5 and 6. The flow of interstellar hydrogen atoms in the solar system is ionized by EUV solar radiation and charge exchange with solar wind protons which accounts for about 80% of the total ionization rate. The resulting gradual decrease of the neutral H density from the upwind region down to the downwind region observed from Ly α intensity measurements allowed the determination of the absolute value of the total ionization rate β for one H atom at 1 AU against ionization. Collected in 1976 and 1977 at five places in the solar system, the measurements are first compared to a model which assumes isotropy of the EUV and solar wind. Strong departures are obvious toward high‐latitude regions, especially when the observer is in the downwind region where the solar wind ionization has had more time to act (cumulative effect). A model was constructed which included a decrease of the ionization rate with heliographic latitude. The adjustment of data allowed for the measurement of the absolute value of the total ionization rate and implies a 50% latitude decrease of the ionization rate due to charge exchange with the solar wind, from βsw = (3.9±0.5) × 10−8; s−1 at the equator to βsw = (2.0±0.5) × 10−8 s−1 at the pole. The corresponding absolute value of the solar wind proton flux is (2.4‐3.6) × 108 cm−2 s−1 at the equator and twice less at the pole if a constant velocity is assumed for the solar wind. Even if the solar wind velocity increases from 400 to 800 km s−1, which would decrease the charge exchange cross section by 25%, there is still a decrease by about 30% of the solar wind mass flux from equator to pole. The Lyman alpha data from Mariner 10 (Kumar and Broadfoot, 1978) had already shown a similar trend in 1974, showing a persistence of the solar wind anisotropy for 3–4 years during a solar minimum. The large‐scale properties of the solar wind mass flux can therefore be monitored at all latitudes by remote sensing of Ly α interplanetary emission, since the solar wind is carving the flow of interstellar H and its anisotropies are ''printed'' on the interplanetary H distribution. With uncalibrated Ly α measurements, an absolute value of the solar wind flux can be determined at all latitudes averaged over a typical 1‐year period.

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

“…Hence, if the flux of the solar wind is higher at the solar equator than at the poles, then the ionization rate in the equatorial region is enhanced and a depression in the distribution of interplanetary hydrogen can be expected (Joselyn & Holzer 1975;Summanen et al 1993;Lallement et al 1995;Bertaux et al 1996a;Summanen 1996). Such a depression, referred to as the heliospheric groove, was indeed observed in some observations of the heliospheric backscatter Lyman-α glow, but it was absent in some others (Ajello 1990;Ajello et al 1987Ajello et al , 1993Bertaux et al 1996b;Lallement et al 1985bLallement et al , 1995Lallement & Stewart 1990;Pryor et al 1992Pryor et al , 1996Pryor et al , 1998. Based on these scarce observations it was hypothesized that the groove appears during solar minimum and disappears during maximum.…”

“…However, already early observations revealed that this was not always the case. It was hypothesized that the non-cylindrical distribution of the Lyman-α heliospheric glow was due to a departure of the solar wind from spherical symmetry (Kumar & Broadfoot 1978, 1979Witt et al 1979Witt et al , 1981Lallement et al 1985b). It was further proposed that the latitudinal structure of the solar wind was connected with the heliospheric current sheet (Bertaux et al 1996a).…”

Interplanetary hydrogen absolute ionization rates: Retrieving the solar wind mass flux latitude and cycle dependence with SWAN/SOHO maps

“…This model is based on the hot model developed by Lallement et al [1985] with corrections for an anisotropic ionization rate and any values for the radiation pressure coefficient . An actual solar line profile derived from the SUMER/SOHO measurements [Lemaire et al, 2002] is used.…”

“…[6] Lallement et al [1985] analyzed the interplanetary Lyman data recorded by the Prognoz 5/6 Lyman photometers in 1976 and 1977. They showed that the Lyman intensity pattern was better represented when an anisotropic ionization flux from the Sun was assumed.…”

Response of the groove in heliospheric Lyman-αglow to latitude-dependent ionization rate

“…Basically, this function is quite complicated since the hydrogen gas is heavily processed by the heliospheric interface (Izmodenov et al 2001). In practice, it is assumed that the gas distribution function at r ∞ is a Maxwellian with a temperature T ∞ , travelling with the bulk velocity vector u B (see also Fahr 1978;Thomas 1978;Wu & Judge 1979;Lallement et al 1985; and a review by Ruciński & Bzowski 1996). The velocity vector u 0 of the atom at r ∞ is calculated numerically by back-tracking the atom from its position (r, θ, δ) inside the heliosphere, where it has velocity vector equal to u, to r ∞ .…”