R. Stamper

A doubling of the Sun's coronal magnetic field during the past 100 years

The solar wind is an extended ionized gas of very high electrical conductivity, and therefore drags some magnetic flux out of the Sun to fill the heliosphere with a weak interplanetary magnetic field,. Magnetic reconnection—the merging of oppositely directed magnetic fields—between the interplanetary field and the Earths magnetic field allows energy from the solar wind to enter the near-Earth environment. The Suns properties, such as its luminosity, are related to its magnetic field, although the connections are still not well understood,. Moreover, changes in the heliospheric magnetic field have been linked with changes in total cloud cover over the Earth, which may influence global climate. Here we show that measurements of the near-Earth interplanetary magnetic field reveal that the total magnetic flux leaving the Sun has risen by a factor of 1.4 since 1964: surrogate measurements of the interplanetary magnetic field indicate that the increase since 1901 has been by a factor of 2.3. This increase may be related to chaotic changes in the dynamo that generates the solar magnetic field. We do not yet know quantitatively how such changes will influence the global environment.

Solar causes of the long-term increase in geomagnetic activity

We analyze the causes of the century-long increase in geomagnetic activity, quantified by annual means of the aa index, using observations of interplanetary space, galactic cosmic rays, the ionosphere, and the auroral electrojet, made during the last three solar cycles. The effects of changes in ionospheric conductivity, the Earths dipole tilt, and magnetic moment are shown to be small; only changes in near-Earth interplanetary space make a significant contribution to the long-term increase in activity. We study the effects of the interplanetary medium by applying dimensional analysis to generate the optimum solar wind-magnetosphere energy coupling function, having an unprecedentedly high correlation coefficient of 0.97. Analysis of the terms of the coupling function shows that the largest contributions to the drift in activity over solar cycles 20-22 originate from rises in the average interplanetary magnetic field (IMF) strength, solar wind concentration, and speed; average IMF orientation has grown somewhat less propitious for causing geomagnetic activity. The combination of these factors explains almost all of the 39% rise in aa observed over the last three solar cycles. Whereas the IMF strength varies approximately in phase with sunspot numbers, neither its orientation nor the solar wind density shows any coherent solar cycle variation. The solar wind speed peaks strongly in the declining phase of even-numbered cycles and can be identified as the chief cause of the phase shift between the sunspot numbers and the aa index. The rise in the IMF magnitude, the largest single contributor to the drift in geomagnetic activity, is shown to be caused by a rise in the solar coronal magnetic field, consistent with a rise in the coronal source field, modeled from photospheric observations, and an observed decay in cosmic ray fluxes.

Long‐term drift of the coronal source magnetic flux and the total solar irradiance

We test the method of Lockwood et al. [1999] for deriving the coronal source flux from the geomagnetic aa index and show it to be accurate to within 12% for annual means and 4.5% for averages over a sunspot cycle. Using data from four solar constant monitors during 1981-1995, we find a linear relationship between this magnetic flux and the total solar irradiance. From this correlation, we show that the 131% rise in the mean coronal source field over the interval 1901-1995 corresponds to a rise in the average total solar irradiance of {\Delta}I = 1.65 +/- 0.23 Wm^{-2}.

Comment on “The IDV index: Its derivation and use in inferring long‐term variations of the interplanetary magnetic field strength” by Leif Svalgaard and Edward W. Cliver

[1] Svalgaard and Cliver [2005] (hereinafter referred to as SC05) use the IDV geomagnetic index to infer the variation of the interplanetary magnetic field strength, B, since 1872. They find that ‘‘B increased by 25% from the 1900s to the 1950s’’ (paragraphs 1 and 24) and that this ‘‘is in contrast to the more than doubling of B during the 20th century obtained from an analysis of the aa index by Lockwood et al. [1999]’’ (paragraph 24). We agree with neither statement. We identify a number of errors and biases in the analysis of SC05, each of which acts in the same direction, namely to reduce the true long-term drift. The key result of Lockwood et al. (hereinafter referred to as LEA99) is a doubling, not in B, but rather in the total coronal source flux which is proportional to the radial IMF component, Br. This is an important distinction which we discus here in section 3. We show (in section 2) that SC05’s own analysis gives a drift in decadal averages in B of 38%, not the ‘‘ 25%’’ that they quote, but we also point out (section 4) that SC05’s method for dealing with data gaps gives rise to a (small) bias and that their regression procedure (which is compared to others in section 5) is not robust (sections 6 and 7), in addition to them not taking into account the distinction between B and Br . We here argue that a simple ordinary linear regression procedure, as used by SC05, is inferior to the method employed by LEA99 (section 9) but does nevertheless show that the IDV index is fully consistent with the doubling in the open solar flux found by LEA99 (section 8).

Predicting Solar Disturbance Effects on Navigation Systems

A variety of operational systems are vulnerable to disruption by solar disturbances brought to the Earth by the solar wind. Of particular importance to navigation systems are energetic charged particles which can generate temporary malfunctions and permanent damage in satellites. Modern spacecraft technology may prove to be particularly at risk during the next maximum of the solar cycle. In addition, the associated ionospheric disturbances cause phase shifts of transionospheric and ionosphere-reflected signals, giving positioning errors and loss of signal for GPS and Loran-C positioning systems and for over-the-horizon radars. We now have sufficient understanding of the solar wind, and how it interacts with the Earths magnetic field, to predict statistically the likely effects on operational systems over the next solar cycle. We also have a number of advanced ways of detecting and tracking these disturbances through space but we cannot, as yet, provide accurate forecasts of individual disturbances that could be used to protect satellites and to correct errors. In addition, we have recently discovered long-term changes in the Sun, which mean that the number and severity of the disturbances to operational systems are increasing.