Solar activity in the approach to maximum of solar cycle 24.

Last week we observed some of the most significant solar activity of solar cycle 24 (SC24), with a series of solar flares and Earth directed coronal mass ejections (CMEs) which were forecast to interact with the near-Earth space environment. This attracted significant media coverage and it was reported that this sequence of events had the potential to cause disruption to Earth’s technological infrastructure, with the possibility of damage to power distribution networks and satellites.

As it happens, the event did interact with the near-Earth space environment, but did not cause significant disruption. Here I’m going to take the opportunity to have a look at this event and give a short summary of some of the ways it could have affected us. I will also briefly analyse how the occurrence of space weather events in SC24 has so far compared with the previous three cycles.

Fig 1. An image of the Sun recorded by NASA's Solar Dynamics Observatory, which displays the X-class solar flare of 06/03/2012 towards the upper left of the image.

This sequence of activity originated from active region AR1492, where two X class flares were released within roughly an hour of each other. Solar flares are the explosive conversion of energy stored in solar magnetic fields into electromagnetic radiation, and the acceleration of charged particles in the solar atmosphere. Flares are designated a classification of either B, C, M, or X according to the observed maximum x-ray flux intensity and each successive class is an order of magnitude larger than the previous. The peak intensities of flares at x-ray wavelengths are distributed as a power law, and X-class events are the rarest, most energetic events. Active region AR1492 was observed to be rumbling with activity ever since the solar rotation brought it into the visible disk, producing some M-class flares and another X-class flare. However the X-class flare on March 6th was particularly noteworthy because it was the second most energetic flare observed in SC24 (a slightly larger flare occurred in Aug 2011). The last flares observed of equal or greater energy were recorded in Dec 2006, in the declining phase of SC23.

The active region also released two Earth directed CMEs associated with these flares, which were estimated to be travelling at approximately 2000 kms-1 and 1700 kms-1, which is far into the high tail of the distribution of CME speeds.  Here is a link to an animation of the first CME, which was recorded by the LASCO instruments. The two outer ring images (blue and red)  are created by obscuring visible light directly from the sun, and detecting visible light reaching the instrument via Thompson scattering off of electrons in the solar corona. Consequently brighter regions represent increased density, and the CME is observed as an increasing “halo” of light as the coronal material is thrown out towards Earth.

How was this event expected to interact with near-Earth space? Unfortunately this will require the introduction of some more acronyms.

Flares and CMEs lead to the generation of solar energetic particles (SEPs). SEP events are observed as large increases (up to 4 orders of magnitude) in the flux of particles in the energy range of roughly 1-100 MeV (1eV = 1.6×10-19 joules). Flare SEPs are accelerated at the footprint of a flare in the solar corona, and typically last a few hours. High speed CMEs propagating through the solar corona and interplanetary space can develop shock waves (analogous to the shock wave of supersonic aircraft) that efficiently accelerate charged nuclei. Shock accelerated SEPs are generally observed to have a higher intensity in near-Earth space, and can last for up to several days. In both instances SEPs can be detected remotely from the acceleration region, as the particles are guided by interplanetary magnetic field lines connecting the source and observer locations.

The high intensity of energetic particle fluxes observed during SEP events pose a significant hazard to satellites, as they may irreparably damage solid state electronics, whilst chronic exposure over time degrades the performance of the photovoltaic cells that power most satellites. Furthermore, SEPs are a concern in the radiation protection of astronauts as well as the crews and passengers of high-altitude aircraft; particularly those taking polar routes, as SEPs are guided here by the geomagnetic field. For example, an SEP event in 1972 was large enough that had any astronauts been outside the protection of the Earth’s magnetosphere, they would have probably received a fatal dose of radiation.

Furthermore, the Earth’s magnetosphere is in dynamic equilibrium with the solar wind, responding to variations in the speed and density of the solar wind flow and the interplanetary magnetic field (IMF) vector.  When the northward component of the interplanetary magnetic field (Bz) is negative, the solar wind is able to efficiently deposit mass and energy into the magnetosphere, causing an intensification of Earth’s ring current. If these temporary enhancements of the ring current are above a certain threshold they are called a geomagnetic storm. This time varying perturbation to the magnetic field at the Earth’s surface can generate geomagnetically induced currents (GICs) in power distribution networks. The generation of large GICs in power grids can cause catastrophic damage to power transformers –as demonstrated by the failure of the Hydro Quebec power system in 1989, due to a large geomagnetic storm.

We can use in-situ measurements of the solar wind to have a closer look at how this event evolved.  Two satellites particularly useful for this are ACE (Advanced Composition Explorer), a satellite situated at the L1 Lagrange point on the Sun-Earth line, and GOES 13 (Geostationary Operational Environmental Satellites) – which as the name suggests is in a geostationary orbit. In Figure 2 panels A and B display the solar wind speed (V­sw) and northward component of the IMF as measured by ACE, and panel C displays the energetic proton flux (F) recorded by ACE (red) and GOES 13 (green). The bottom panel shows the variation of the disturbance storm time (Dst) index of geomagnetic activity. The Dst index is constructed to be sensitive to changes in Earth’s ring current, and is therefore suitable for diagnosing geomagnetic storms. Time zero on the plots is set to coincide with the X-flare onset.

Figure 2. Panels A-D show the variation of the solar wind speed, northward component of the IMF, energetic proton flux, and Dst geomagnetic index after the onset of the X-class flare on 06/03/2012.

An SEP event commenced a few hours after the X-flare occurrence. The persistence of this event over many days implies that these are predominantly shock accelerated SEPs. The SEP event was less intense at GOES relative to ACE, which is possibly due to the magnetosphere deflecting some of the SEPs and shielding GOES, but the separation of the satellites and instrumental differences will be factors too. According to an SEP event database produced by NOAA, this is the largest SEP event since 2003, with the 11th largest peak energetic proton flux in a database of 235 events spanning 1976-present.

At approximately t = 3 days there appears to be a discontinuity in the solar wind speed. This sharp increase in the solar wind flow speed is consistent with the picture of an SEP accelerating shock, ahead of an Earth directed CME, sweeping over ACE, although it would require further work to confirm this.

Thresholds of Dst <50 nT and DsT<100 nT are often used to define “moderate” and “strong” geomagnetic storms (DsT<200 for a “severe” storm). Panel D therefore shows that shortly after the X-flare onset a moderate geomagnetic storm took place, with a brief  period of recovery before the commencement of a strong geomagnetic storm after t = 2.  Comparing the progression of the geomagnetic activity with the variation of Bz shows that the geomagnetic disturbance intensifies after periods when Bz is more negative – the condition required for effective mass and energy deposition from the solar wind to the magnetosphere.

This event attracted a lot of attention because it was one of the few space weather events of SC24. Although this event produced a large SEP event, and a strong geomagnetic storm, it is not the most remarkable of events. To me, what seems more remarkable, is the absence of events in SC24 – such an absense that a fairly typical event becomes noteworthy.

As well as producing a database of SEP events NOAA also produce a catalogue of solar flares and we can use these to compare how the occurrence of SEPs and X-class flares in SC24 compares to previous cycles. Some recent work by Owens et al. has shown that we are probably approaching the maximum activity of SC24, which is predicted to occur sometime in 2012, with the best estimate of the current phase of the solar cycle being 112 degrees.  In Table 1 the total occurrence of SEP events and X-class flares for the phase period 0-112 degrees, for SC21-24, are listed.

SC21 SC22 SC23 SC24
NSEP 14 20 19 10
Nx-flare 38 41 29 15

There have been roughly half as many events in SC24 than in the three previous cycles. Could this be linked to predictions that the grand solar maximum of the space age is ending, with a predicted 8% chance of a return to Maunder minimum levels of activity in the next 40 years? Or could it just be a statistical fluctuation due to counting small numbers of events? I’m not sure yet, but I do think the next few years will be a really interesting time to study the Sun.

N.B The solar wind data, Dst geomagnetic index, and SEP and flare data used in this post were obtained from NOAA’s Space Weather Prediction Center and I’d like to thank Simon R. Thomas for help compiling this post.

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