Predicting arrival of solar eruptions using heliospheric imaging

The Sun unleashes a gigantic cloud of plasma and magnetic field into interplanetary space. These so-called coronal mass ejections (CMEs) are main causes of space weather disturbances, including colorful aurora and various hazards to modern technology in space and on ground. Predicting accurately space weather conditions is becoming increasingly important.

Even the basic question whether a given CME will hit the Earth or not remains surprisingly difficult to answer. Wide angle heliospheric imaging is a compelling future way for systematic space weather forecasting. The accuracy of the methods based on this instrumentation to predict CME arrivals has however not been previously tested using a large sample of events. In the recently published Space Weather paper led by Chris Möstl from Space Research Institute Austrian Academy of Sciences we validate a self-similar expansion model to predict the CME impacts using eight years of heliospheric imaging observations from the twin STEREO spacecraft and the data from five in-situ spacecraft. The paper is done as part of the HELCATS  project and we used over 1300 CMEs extracted from the HELCATS catalogues.

Wide-angle heliospheric white-light imaging can detect coronal mass ejections until the Earth orbit and beyond. The figure is from the HELCATS catalogues.

We found that approximately one third of the CMEs that were predicted to hit a certain location were associated with a clear interplanetary CME. This percentage of “hits” may seem low. Our study however sets an important baseline for utilizing systematically heliospheric imaging to predict CME arrivals. We present various ways to improve the technique and decrease the number of “false alarms”. Instead of using the same fixed angular width for all CMEs, their more realistic sizes could be determined from the modeling. Also a significant fraction of “misses” are likely the cases where a CME arrived as predicted but produced less clear interplanetary signatures.

The up-coming Solar Orbiter and Parker Solar Probe will carry wide-field imagers. A space weather mission to the Lagrangian point L5 is also actively planned. A heliospheric imager onboard such a mission would provide continuous monitoring of the heliosphere between the Sun and Earth. Our study gives further support that this is an optimal location for predicting Earth-directed CMEs using heliospheric imaging.

Mystery of Solar Flux Ropes

Mystery of Solar Flux ropes

Coronal mass ejections show twisted magnetic field structure. Such tubes of twisted magnetic fields are called as flux ropes and they can store plenty of free magnetic energy in the corona. The questions when and how the flux ropes form have puzzled scientists for decades. In some cases magnetic fields may survive twisted deeper from the Sun, but mostly the flux ropes that are integral part of erupting coronal mass ejections are believed to form in the solar corona. However, it is not clear yet whether the flux rope typically forms prior to the eruption or during the eruption process. The plasma instabilities and processes that trigger and drive the eruption are also largely unknown.

In this Solar Physics paper led by Alexander James from Mullard Space Science Laboratory we combined a wide variety of observations (e.g., EUV, radio and magnetograms) to analyse the formation and eruption of a flux rope on June 14, 2012. We found that magnetic reconnection formed the flux rope a few hours before its eruption in the corona. This was consistent also with plasma composition analysis suggesting the existence of coronal plasma in the flux rope. This eruption originated from an emerging active region where vortical motions of the flux rope footpoints caused it to rise and become hence unstable. The paper also highlights the importance of combining different observations for shedding light on how solar eruptions form.


Left) Sigmoidal Extreme Ultraviolet structure signalling the flux rope, Right) associated coronal mass ejection detected by coronagraph onboard STEREO-A

Tracking Solar Storms: Recap of the HELCATS project

Tomorrow we will have the final HELCATS meeting in the beautiful city of Vienna. HELCATS was a three-year consortium project funded by the European Commission 7th Framework Programme. Eight institutes or universities from seven European countries were involved. The project was led by Richard Harrison from Rutherford Appleton Laboratory, UK. We had monthly telecons, held face-to-face project meetings twice per year and organised three open science meetings. Several science visits were also done during the project.

The core of our project was to use wide-angle heliospheric imagers onboard the twin STEREO spacecraft to study solar eruptions and the solar wind (see figure below). Heliospheric imaging is currently one of the only ways to bridge the huge gap between solar and direct solar wind observations.

Over the past three years we worked hard to build various catalogues for the scientific community. All the catalogues are now uploaded to our project webpage and they are ready to use.

The catalogue of coronal mass ejections (CMEs) identified manually from the heliospheric imager data is the “Master catalogue” and first of its kind. We used also widely other observations. For example, you can find the catalogues that give key CME parameters based on the geometric modelling of coronagraphic data and in-situ reconstructions, information on CME sources and associated radio waves. We also produced a catalogue of solar wind stream interaction regions that could be traced with heliospheric imagers. The catalogue I find particularly useful in my own studies links solar, heliospheric and in-situ CMEs.

Although the project ends, the work grounding on HELCATS will continue. One of the best things in participating in such a big international project was to get to know better and work with many great scientists in the field. And young scientist can establish their scientific network. Below you can find a summary video I made of our final open science meeting.

Visit HELCATS webpage

Left) STEREO twin spacecraft, Middle) coronal mass ejection seen with heliospheric imagers, Right) my student Erika extending her network with HELCATS scientists

HELCATS_EGU from Emilia Kilpua on Vimeo.

Solar wind and strong space weather storms

Since 1960s we have known that southward interplanetary magnetic fields and fast solar wind are behind every significant space weather disturbance. These conditions allow magnetic reconnection opening the dayside magnetopause and transferring effectively solar wind plasma and momentum into the Earth’s magnetosphere.

Stronger and faster solar wind does not however necessarily mean stronger space weather storms. This non-linearity in the response of our near-space environment is known as the saturation of the potential across the polar cap. Many theories have been presented to explain why polar cap potential saturates, but none of them works for all solar wind conditions.

In this paper we just published in Geophysical Research Letters we examine how the saturation depends on solar wind conditions, in particular the solar wind dynamic pressure. We examined three decades of near-Earth solar wind measurements. Our key finding is that the high solar wind dynamic pressure can prevent the polar cap potential from saturating. This means that the largest geospace storms occur when dynamic pressure is high.

But what structures in the solar wind can provide such conditions? The best candidates are the magnetosheaths of coronal mass ejections (CMEs) that are composed of the turbulent plasma compressed at the CME-driven shock waves. From our previous works we indeed know that the sheaths are very efficient drivers of space weather storms and that their couple with the magnetosphere stronger than the CME flux rope itself.

Link to the paper:

Lasikattojen läpi, tähtiä kohti

Naistenpäivän 8.3 kunniaksi Hufvudstadsbladet haastatteli miesvaltaisilla aloilla työskenteleviä naisia (ks. linkki alla). Avaruusfysiikan parissa Suomessa on ollut jo pitkään erittäin hyvä naisedustus, myös niillä kaikkein korkeimmilla paikoilla. Tilanne ei ole kuitenkaan yleisesti näin hyvä luonnontieteiden alalla. Naisten osuus laskee merkittävästi mitä ylemmäksi akateemisella urapolulla edetään. Helsingin yliopistolla fysikaalisten tieteiden koulutusohjelmaan hyväksyttyjen naisten määrä on pysynyt pitkään suhteellisen samalla tasolla, 30-35 prosentin välillä. Laitoksen professoreista kuitenkin vain noin 15 prosenttia on naisia. Tässä on se kuuluisa lasikatto.

Miksi sitten niin useat naiset törmäävät lasikattoon? Löytyykö syy hyväveli-verkostoista? Vai onko menestyksekkään uran ja perhe-elämän yhdistäminen mahdoton yhtälö? Naisilla on kuulemma myös taipumus aliarvioida omia kykyjään, kun taas miehet paukuttelevat henkseleitään huomattavasti herkemmin. Syitä on varmasti monia ja kaikkeen ei voi itse vaikuttaa, mutta ainakin me luonnontieteiden alalla jo pitkän matkan kulkeneet voimme pyrkiä olemaan mahdollisimman paljon positiivisena esimerkkinä. Useat tutkimukset ovat korostaneet, että esikuvilla, sekä kannustavilla vanhemmilla ja opettajilla on suuri merkitys. Ja luonnontieteiden parissa vasta aloittaville tytöille/naisille vinkkinä, että uskokaa itseenne. Parempia virkoja ei ainakaan saa jos niitä ei uskalla hakea. Fifty-fifty prosentit eivät tässä välttämättä ole se asian ydin, mutta ainakin se, että jokaisella olisi mahdollisuus kiinnostua asioista ja toteuttaa niitä ilman yhteiskunnan asettamia ennakko-oletuksia. Ja että kaikilla olisi yhtäläiset mahdollisuudet edetä urallaan.

Transition through the magnetosheath matters for space weather

Magnetic clouds are gigantic helical flux ropes in interplanetary space. They originate from the Sun as violent eruptions of plasma and magnetic field, called coronal mass ejections. Magnetic clouds are one of the key causes of significant space weather storms at the Earth as they often embed strong and southward magnetic fields. Before reaching the Earth’s magnetic environment, magnetic clouds  move past the bow shock and the magnetosheath.  This can alter significantly their magnetic structure and change their ability to drive geomagnetic disturbances. In our new Journal of Geophysical Research  paper we study the transition of 82 magnetic clouds from the solar wind to the Earth’s magnetosphere using a magnetohydrodynamic (MHD) model and observations from THEMIS, Double Star, GEOTAIL, and Interball. We found that the largest changes in their magnetic structure occurred when the bow shock was quasi-parallel, i.e., when kinetic processes dominate. In such a case the MHD model is not sufficient to capture the transition. Our results also emphasise the importance of the east-west magnetic field component in controlling the ability of a magnetic cloud to drive magnetospheric storms. This has important implications for forecasting space weather as all those spacecraft  that provide continuous solar wind measurements are  located upstream of the bow shock.

Check the full paper from


Aurinkomyrskyjen arvoitusta ratkaisemassa

Talouselämän jutussa kerron tutkimuksestani ja siitä miksi luonnontieteitä kannattaa opiskella ja jopa harkita uraa niiden parissa. Luonnontieteiden opiskelu avaa monenlaisia ovia työmaailmassa. Töissä oppii jatkuvasti uutta ja jokainen päivä on taatusti erilainen. Bonuksena työ kuljettaa välillä hienoihin paikkoihin ympäri maailmaa. Lehtiversiossa selviää myös se, mikä Havaijissa kiehtoo.


Magnetic structure of solar eruptions

Our paper on estimating the intrinsic magnetic structure of coronal mass ejections (CMEs) was published today in Solar Physics. This topic is highly important for solar and space weather research. Without knowledge of the magnetic structure in erupting CMEs we cannot predict space weather and cannot study properly how CMEs initiate and evolve in the corona and in interplanetary space. Currently, the magnetic field in the corona cannot be measured reliably (as corona it is so hot and tenuous). The only way to get information in advance is through modelling or using in-direct proxies based on remote-sensing observations.

In this paper we estimate the magnetic structure for two active region CMEs. Both turned out to be more complex than we initially anticipated. One of these events erupted in two phases and the other one started quite high in the corona. In addition, active region CMEs usually lack clear filament counterpart. This means that we could not use fully indirect proxies relying on filament characteristics. However, by combining a wide range of different techniques based on multi-wavelength solar observations we were able to estimate the magnetic structure for these CMEs. See our paper in Solar Physics and arXiv

Here are some nice pictures of the eruptions we studied. On left is the erupting flux rope in Extreme Ultraviolet from Solar Dynamics Observatory and on right the CME captured in white-light coronagraph onboard STEREO-A spacecraft.

Kvanttifysiikkaa ja klassista sähkömagnetismia

Peruskurssilla tulee perustavanlaatuisia kysymyksiä. Miten sähkökentät ja magneettikentät ihan oikeasti syntyvät? Miksi ne heikkenevät kääntäen verrannollisesti etäisyyden neliöön? Mikä on kvanttimaailman ja klassisen sähkömagnetismin yhteys? Mitä virtuaalisista fotoneista pitäisi oikein ajatella? Pyysin kvanttifysiikkaekspertti Aleksi Vuorisen keskustelemaan kanssani ensimmäisen vuoden sähkömagnetismin luennolle. Taululla nähtiin mm. Feynmanin diagrammi ja polkuintegraali. Alunperin suunniteltu 45 minuuttinen venyi pitkälle tauon yli seuraavalle luennolle ja kysymyksiä tuli runsaasti. Ja ensimmäistä kertaa kuulin luennolla aplodit.