Why exoplanets should have ionospheres and brown dwarfs chromospheres

Do exoplanets have an ionosphere? What does a brown dwarf need to form aurorae even without a companion? Isabel Rodriguez-Barrera and colleagues, including Christiane Helling and former member Craig Stark from the LEAP group, investigated whether it is possible to create a magnetized plasma, a medium composed of positive and negative charges but with an overall neutral electric property. The production of magnetized plasma would allow the creation of ionospheres and electromagnetic phenomena such as aurorae.

The ionosphere is the upper part of a planetary atmosphere created by the ionizing effects of stellar UV and X-ray radiation. Its importance inheres in atmospheric electricity and radio wave propagation, but also the shielding of the inner atmosphere from stellar UV radiation. The following study showed what conditions a planetary atmosphere (that is an atmosphere from a planet or a planet like object like a brown dwarf) should fulfil to produce an ionosphere or, in case of brown dwarfs, a chromosphere, such as the spectacular example of the solar chromosphere.

Radio, X-ray and Hα emission from brown dwarfs have been observed in the recent years (e.g. 2MASS J10475385+2124234 by Route & Wolszczan 2012; 2MASS J13153094-2649513AB by Burgasser et al. 2013). Similar observations are not yet available for extrasolar planets. In case of the Sun, observations of these emissions (radio, X-ray and Hα) are tracers of the solar chromosphere. These observations suggest that brown dwarfs contain ionized gas and host very strong magnetic fields, which are both needed to explain, for example, the radio emission. The aim of our study is to identify ultra-cool objects (with effective temperatures less than ~3000 K; Fig. 1.) that are most susceptible to processes leading to instabilities that trigger the emergence of strong plasma, a neutral state of matter composed of equal number of positive and negative ions.

Our theoretical work proposes a method of analysing the ionization and magnetic coupling state of objects with ultra-cool atmospheres. Our particular interest focuses on late M-dwarfs, brown dwarfs and giant gas planets.

Figure1. M-dwarfs, brown dwarfs and giant gas planets in comparison. Teide 1 is an example for a late M-dwarf, GD 165B for a cloud-forming brown dwarf of spectral type L, Gliese 229B is a cooler cloud-forming brown dwarf of spectral class T, and Jupiter is the example for a giant gas plane.

Figure 1. M-dwarfs, brown dwarfs and giant gas planets in comparison. Teide 1 is an example for a late M-dwarf, GD 165B for a cloud-forming brown dwarf of spectral type L, Gliese 229B is a cooler cloud-forming brown dwarf of spectral class T, and Jupiter is the example for a giant gas plane.

To determine the fraction of atmosphere that can be ionized, first Rodriguez-Barrera et al. consider thermal ionization only. Thermal ionization results from collisions between the gas particles according to the local gas temperature, therefore here we do not consider external ionizing affects from companions. Sources of possible irradiation are the host star in the case of planets, and a white dwarf in the case of a white dwarf – brown dwarf binary (for example WD0137-349B, Casewell et al. 2015). Such external affects can be later compared to the results of our reference study. We use the Drift-Phoenix model atmosphere grid where the local atmospheric structure is determined by the following global parameters: Teff (effective temperature), log(g) (surface gravity) and [M/H] (metallicity).

Rodriguez-Barrera et al. show that ultra-cool atmospheres with high Teff , or with high metallicity and low log(g) have large fraction of atmospheric volume where plasma processes occur, and so they are the best candidates for radio, X-ray and Hα emissions, observed from various objects as was mentioned above.

Figure 2. The volume fraction of the atmosphere that is thermally ionized, V^th_gas/V_atm, for f_e>10-7 and for M-dwarf, brown dwarf and gas giant planet atmospheres. f_e measures the extent to which a gas is ionized. (Rodriguez-Barrera et al. 2015)

Figure 2. The volume fraction of the atmosphere that is thermally ionized, V^th_gas/V_atm, for f_e>10-7 and for M-dwarf, brown dwarf and gas giant planet atmospheres. f_e measures the extent to which a gas is ionized. (Rodriguez-Barrera et al. 2015)

M-dwarfs have a considerable degree of ionization throughout the whole atmosphere, the degree of thermal ionization for a L-dwarf is low but high enough to seed other local ionization processes like Alfven ionization (see Stark et al. 2013) or electrostatic discharges, such as lightning, as seen on Fig. 2.

We show that the first criterion to form chromospheres, ionospheres or an aurora on an extrasolar planets or brown dwarf, namely a small but sufficient degree of ionization, can be fulfilled by thermal ionization alone without the need for additional processes. But is it possible to magnetise this ionized plasma? The second part of our study says yes! The results also give an idea of how well the different types of atmospheres can be magnetized (Fig. 3): The minimum threshold for the magnetic flux density required for electrons and ions to be magnetised is smaller than typical values of the global magnetic field strengths of a brown dwarf and a giant gas planet. This means the ionized plasma inside the atmosphere can be magnetised quite easily. A considerably lower magnetic flux density is required for magnetic coupling of the atmosphere in the rarefied upper atmosphere than in the dense inner atmosphere, meaning magnetising the plasma in the upper atmosphere is easier than in the inner parts of the atmosphere. The magnetic coupling works equally for electrons and atomic ions like Mg+ and Fe+ (Fig. 3).

Figure 3. The magnetic flux density required for electrons, Be (lower set of lines), and ions, Bi (upper set of lines), to be magnetically coupled to a background magnetic field in the object (B=10 G - giant gas planets (GP), B=103 G M-dwarfs (MD), brown dwarfs (BD); black horizontal/vertical lines). If B>Bi (or B>Be) the gas is magnetized by the external magnetic field B. (Rodriguez-Barrera et al. 2015).

Figure 3. The magnetic flux density required for electrons, Be (lower set of lines), and ions, Bi (upper set of lines), to be magnetically coupled to a background magnetic field in the object (B=10 G – giant gas planets (GP), B=103 G M-dwarfs (MD), brown dwarfs (BD); black horizontal/vertical lines). If B>Bi (or B>Be) the gas is magnetized by the external magnetic field B. (Rodriguez-Barrera et al. 2015).

To sum it up, our results suggest that it is not unreasonable to expect ultra-cool atmospheres (M-dwarfs and L & T brown dwarfs) to emit Hα or even in radio wavelength. We showed that, in particular, the rarefied upper parts of the atmospheres fulfil quite easily the plasma criteria despite having low degrees of ionization also in the case of giant gas planets. Therefore the results suggest that an ionosphere may emerge also in brown dwarf and giant gas planet atmospheres, and that the built-up of a chromosphere on brown dwarfs is likely too. Both effects will contribute to atmospheric weather features and to space weather occurrence in extrasolar, planet-like objects. An interesting result is that ultra-cool atmospheres could also drive auroral emission without the need for a companion’s wind (e.g. aurora on Earth triggered by solar wind) or an outgassing moon (e.g. aurora on Jupiter is triggered by its outgassing moon Io).

fig4

Figure 4. The dominating thermal electron donors for a subset of effective temperatures for log(g)=3,0 and solar element abundances, against the local gas pressure (Rodriguez-Barrera et al. 2015).

We further studied which of the gas species might be the best electron donors. Na+, K+ and Ca+ are the dominating electron donors in low-density atmospheres (low log(g), solar metallicity) independent of Teff. Mg+ and Fe+ dominate the thermal ionization in the inner parts of M-dwarf atmospheres. Molecules remain unimportant for thermal ionization. Chemical processes (e.g. cloud formation, cosmic ray ionization) that affect the abundances of Na, K, Mg, Ca and Fe will have a direct impact on the state of ionization in ultra-cool atmospheres.

For more details check out the original paper on ADS:

 Rodríguez-Barrera, I.; Helling, Ch; Stark, C. R. and Rice, A. M. 2015, MNRAS 454, 3977

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Electrification in dusty atmospheres inside and outside the solar system, Pitlochry, 8-11 Sep. 2014

The workshop ‘Electrification of dusty atmospheres inside and outside the solar system’ hosted by the LEAP Group took place in Pitlochry, Scotland. The cross-disciplinary nature of the workshop attracted scientists from fields of plasma physics, volcanology, meteorology, and astrophysics from all over the world.

The meeting started with a welcome barbeque on Sunday evening: people were talking in small groups, catching up with old friends and meeting new colleagues.

The welcome barbeque in the garden of the hotel

_DSC1519 The welcome barbeque in the garden of the hotel (Credit: Rubén Asensio Torres)

On Monday morning Christiane Helling summarized the scientific idea that lead to the organization of this workshop: she talked about the benefits of the meeting for both astrophysicists and scientists from other fields. She also introduced a new proceeding idea, which is planned to be published in Surveys of Geophysics.

The first talk was given by Alan Phelps who discussed laboratory studies of crystalline-like ordered structure in dense dusty plasmas, with the potential to investigate similar behaviour in substellar atmospheres. In this context, the exciting possibility exists of identifying a unique observable signature associated with plasma crystals that could be used to diagnose the charged environment.

The difficulties of the inter-disciplinary nature of the workshop appeared right after the first talk when it turned out that the definition of ‘dust’ is not the same in every field. However, after discussing the issue, the speakers and participants quickly got used to the fact that most of the people are from a different field than they are and explained their fields in a way, which was understandable for everyone.

Keri Nicoll and Corrado Cimarelli gave exciting talks on volcanic lightning. Nicoll gave an overview on the different charging mechanisms in volcanic plumes and reported that broad particle size distributions of volcanic ash clouds are more susceptible to triboelectric charging, which give an analogy to substellar clouds with atmospheric regions with the appropriate particle size distribution. Cimarelli described a laboratory experiment where they reproduced volcanic lightning strikes, and explained how the particle size and distribution affects the charge separations on plumes.

Euan Bennet’s talk on isolating different sized bacteria using electrostatic disruption of water droplets was an interesting part of the conference. It showed some of the unexpected applications that can arise from the study of aerosol electrification.

During the afternoon session Ute Ebert introduced us into the mechanism of lightning development and gave an overview of streamer propagation. The following talks were about Transient Luminous Events (TLEs) such as sprite modelling and the possibility of TLE initiation on gas giant planets like Jupiter.

The afternoon ended with the poster pop-up, where each poster presenter was given one minute to advertise his or her work, which was followed by the poster session itself. Delicious pretzels and Guinness accompanied the session.

_DSC1609

_DSC1623 Poster session (Credit: Rubén Asensio Torres)

On Tuesday we started with a very interesting talk by Farideh Honary on Lunar dust charging and how this can affect future (and past) landing missions. Karen Aplin introduced us a similar approach but with asteroids. She raised the question of what would happen if a, possibly, oppositely charged landing spacecraft (negatively charged) and the surface of an asteroid (positively charged) interact with each other and showed a model of how the electrostatic effects can be best measured in situ.

The afternoon session started with Ian Dobbs-Dixon’s presentation on dynamical modelling of the atmospheres of tidally locked hot Jupiters. Michael Rycroft introduced the audience to the conditions a planet would need in order to host a global electric circuit.

In the evening we had the workshop dinner in the hotel. In a short dinner speech, Christiane Helling also thanked all the participants for their exciting contributions to the workshop. Towards the end of the dinner Craig Stark announced the winners of the poster contest, Graham Lee and Karen Aplin. Congratulations!

Wednesday was the day of brown dwarfs (BDs) and ionization processes. Sara Caswell talked about two White Dwarf–Brown Dwarf systems and showed how different the spectra of the day and night side of an irradiated BD can be. Irena Vorgul gave a talk on how flash ionization processes (such as lightning) could be detected through cyclotron maser emission going through the affected atmospheric volume. Craig Stark summarized the concept of the LEAP Project, then talked about the basics of Alfvén ionization, a process where a low density magnetized plasma is hit by a high speed flow of neutral gas. He then talked about the possibility of creating prebiotic molecules (like glycen) on the surface of dust particles in plasmas. An impressive talk was given by Takayuki Muranushi, how proposed to use ion lines width for detection lightning occurring within protoplanetary disks.

On the last day of the workshop we learnt a lot about cosmic ray (CR) air showers and their ionizing effects. However, due to a change in the schedule, the first talk was about multi-wavelength observations of BDs given by Stuart Littlefair. He showed that consistent cyclotron emission detection shows very good correlation with optical observations, suggesting an aurora-like mechanism for the radio emission. There is though some variation in radiated power for different periods of rotation, which might also be attributed to undergoing transient processes in the atmosphere (like lightning).

Alan Watson talked about the work at the Pierre Auger Observatory, an ultra-high-energy CR detector in Argentina. He showed us an unusual phenomenon observed by multiple detectors and asked the opinion of the audience on the topic. Large variety of ideas came including possible lightning events, and military missile activity as well. Although the question has not been answered unequivocally, the response from the audience showed how beneficial such a multi-disciplinary meeting can be for the different scientific fields. Paul Rimmer went into the details of CR ionization in BD atmospheres and proposed the possibility of using Jupiter as a giant gamma-ray detector through the extensive CR air showers occurring in its atmosphere.

The last talk of the day and the workshop was given by Scott Gregory who showed us how stellar magnetic fields can affect the habitability of a planet orbiting that star. He also pointed out that the magnetic field structures differ for different stars.

The afternoon was rounded off with a whiskey tour and tasting in the Blair Atholl Destillery where we learnt a lot on how whiskey is made, what are the main ingredients, how is the alcohol content regulated and how much time the infusion spends in the barrels.

A few of the participants had the opportunity to tour the Blair Castle and its extensive grounds on Friday. The fresh apples and pears from the trees in the Hercules garden were especially enjoyable.

On the whole the workshop was a great experience for all of us, the talks were very diverse still related to our work in the LEAP Group. All speakers made great efforts to allow the audience to appreciate their contribution to the workshop’s theme. We had a great opportunity to meet scientists from other fields and discuss our projects, concerns, works with them.

We would like to thank all of the participants for their contribution to the success of the workshop. The high quality of the talks and posters gave an insight for the audience into the different disciplines.

 

Participants of the workshop (Credit: Rubén Asensio Torres)

Participants of the workshop (Credit: Rubén Asensio Torres)