The interplanetary magnetic field: radial and latitudinal dependences

TL;DR

This study analyzes spacecraft data to understand how the interplanetary magnetic field's radial and latitudinal components vary with distance and latitude, revealing a r^-5/3 dependence and latitudinal increases near 30 degrees.

Contribution

It provides new insights into the radial and latitudinal dependence of the IMF, challenging the traditional r^-2 model and exploring the bimodal distribution of solar wind speed.

Findings

|Br| decreases as r^-5/3 in the ecliptic plane

|Br| and IMF strength increase near 30 degrees latitude

Solar wind speed anti-correlates with solar activity at high latitudes

Abstract

Results of the analysis of spacecraft measurements at 1-5.4 AU are presented within the scope of the large-scale interplanetary magnetic field (IMF) structure investigation. The work is focused on revealing of the radial IMF component (Br) variations with heliocentric distance and latitude as seen by Ulysses. It was found out that |Br| decreases as ~r^-5/3 in the ecliptic plane vicinity (10 deg. of latitude). This is consistent with the previous results obtained on the basis of five spacecraft in-ecliptic measurements (Khabarova, Obridko, 2012). The difference between the experimentally found (r^-5/3) and commonly used (r^-2) radial dependence of Br may lead to mistakes in the IMF recalculations from point to point in the heliosphere. This can be one of the main sources of the 'magnetic flux excess' effect, which is exceeding of the distantly measured magnetic flux over the values obtained through the measurements at the Earth orbit. It is shown that the radial IMF component can be considered as independent of heliolatitude in a rough approximation only. More detailed analysis demonstrates an expressed |Br| (as well as the IMF strength) increase in the latitudinal vicinity of 30 deg. relative to the ecliptic plane. Also, a slight increase of the both parameters is observed in the polar solar wind. The analysis of the latitudinal and radial dependences of the Br distribution's bimodality is performed. The investigation has not revealed any dependence between Br and the solar wind speed V. Meanwhile, the two-peak distribution of the solar wind speed as measured by Ulysses is a consequence of a strong latitudinal and solar cycle dependence of V. It is shown that the solar wind speed in high latitudes (above 40 deg.) anti-correlates with a solar activity: V is maximal during the solar cycle minima, and it has a minimum at the maximum of solar activity.