My research ranges from the very small-scale, understanding the quantum mechanical properties of energetic transitions between different atomic states, to the very large-scale, studying the generation of the heliosphere that envelops the solar system and influences the near-Earth space environment.
Recently, I have mainly been working in the following areas:
Origin and formation of the solar wind
Fast and slow winds flow from the Sun, fill the entire solar system with hot gas, and affect the Earth’s magnetic environment. The fast wind streams at more than 700 km per second, and originates from the Sun’s polar regions. The source regions of the slower wind, only(!) 400 km per second, have long been debated. The slow and fast winds have a different chemical composition signature, which is an important clue for attempts to identify the sources.
By combining new measurements from the Hinode satellite with a solar magnetic field model, we have attempted to identify the most likely sources by constructing a map of the whole Sun that shows areas where electrified gas with the same chemical composition as the slow wind is flowing out from the solar atmosphere. The image on the right shows a map made from observations taken in October 2015. The slow wind sources are shown in red, overlaid on an ultraviolet image of the Sun (blue).
We have analyzed an older one of these maps (made from observations in January 2013) in detail, and found that the most significant sources are outflows from the edges of sunspots and dark holes in the corona at equatorial and mid-latitudes. The results were published in Nature Communications and featured in The Economist.
Sources of solar energetic particles
When solar activity is high, large flares and eruptive events (called coronal mass ejections – CMEs) can occur in the solar atmosphere. CMEs can accelerate high energy particles (electrons, protons, and heavy ions), and if these are directed towards Earth, they are potentially dangerous for our technological infrastructure. The electronics on space satellites can be damaged, and there is a radiation risk to crewed spaceflight and even commercial aviation.
Usually we think of two types of solar energetic particle (SEP) events: impulsive and gradual. Impulsive (short duration bursts) tend to be associated with solar flares and jets. The most hazardous SEPs, however, arrive during long duration gradual events. The sources of these events are still debated.
One unique fingerprint of these events is that the chemical composition is unusual. In particular, the amount of Sulfur in SEPs is less than in the solar wind, and sometimes much less.
Using measurements made by the Wind spacecraft near Earth, we attempted to trace this fingerprint back to its source on the Sun for several gradual SEP events observed in January 2014 when solar activity was high. We used the Hinode satellite – with a similar technique as discussed above – to show that gas trapped low in high temperature (7 million degree) coronal loops (see the image to the right), by strong magnetic fields, develops the SEP composition fingerprint. This material is continuously released and later accelerated by eruptive events.
The results were published in Science Advances.
The paper also includes coronal magnetic field measurements using a rare magnetically induced transition, and also a technical discussion of the location where the mechanism that causes differences in chemical composition operates.
Heating of the solar corona
The outer layer of the Sun’s atmosphere, the corona, has a peak temperature of over one million degrees. Understanding how the corona is heated is one of the most significant unsolved problems in astrophysics.
The hot gas in the atmosphere is organized into coronal loops by the magnetic field (see the ultraviolet image to the right). Understanding the physical properties of these loops would provide important constraints for theoretical models.
One of the most widely accepted ideas is that they are composed of many fine magnetic threads. Energy released when the magnetic threads break and reconnect heats the gas.
This is expected to happen on very small spatial scales. The actual scale of these loops, however, is unknown, so we do not know if the properties we measure in these loops correspond to single loop structures, or the combined properties of multiple threads.
By examining observations from Hinode, the Solar Dynamics Observatory, and Hi-C, we have argued that these loops are organized on much larger scales than previously thought, providing a significant challenge to current theories. The results were published in The Astrophysical Journal in 2012 and 2013.