PROTON OBSERVED FROM THE GROUND Marina Galand (Center for Space Physics, Boston University, MA, USA; [email protected])

1. INTRODUCTION Auroral particle precipitation is an important energy source which affects the electrodynamic properties, dynamics, thermal structures, as well as the constituent distribution at the high latitude mesosphere and lower thermosphere region. Most of the auroral energy is carried into the by energetic electrons. There are, however, occasions when proton precipitation in the keV range represents a significant part of the energy input upon the auroral ionosphere. This is the case at the equatorward edge of the afternoon/evening auroral oval and in the cusp [e.g., Hardy et al., 1989]. In these regions, proton precipitation makes a significant contribution to, even dominates at times, the ionization [e;g, Basu et al., 1987; Galand et al., 2001] and the visible and far- ultraviolet aurora [Frey et al., 2001; 2002; Lummerzheim et al., 2001; Galand et al., 2002; Ivchenko et al., 2004]. Energetic electrons and protons do not interact in the same way within the atmosphere [e.g., Strickland et al., 1993; Galand and Richmond, 2001; Galand and Lummerzheim, 2004]. It is thus crucial to separate the electron and proton components of the precipitation in order to correctly assess ionospheric conductivities, heating, and composition changes, which is one of the important scientific issues of polar aeronomy, as stated by Coupling, Energetic, and Dynamics of Atmospheric Regions (CEDAR)/Phase III. In addition, as protons retain more efficiently the large scale structure than electrons, imaging the proton aurora is an excellent probe for investigating magnetospheric substorms and -ionosphere coupling processes [e.g., Rees, 1989; Samson et al., 1992; Deehr and Lummerzheim, 2001; Immel et al., 2002; Mende et al., 2003; Zhang et al., 2004]. H emissions resulting from excited H atoms produced within the incident proton beam are a unique signature of proton precipitation. They are Doppler-broadened and –shifted, as the emitting H atoms, produced by charge-exchange reactions between energetic protons and atmospheric species, are energetic. In the altitude region where precipitating particles deposit their energy, the ambient H atom density is too low to produce any significant amount of auroral emissions from excitation by energetic particles, so the contribution from precipitating electrons or secondary electrons is negligible. Proton aurora is diffuse owing to the neutral component of the energetic beam whose path, independent of the magnetic field configuration, produces a spreading of the incident proton beam. In this paper, we use the terms “electron aurora” and “proton aurora” exclusively to distinguish the type of precipitating particles. Both types of aurora have the full spectrum of auroral emissions produced by atmospheric atoms and molecules. The proton aurora has in addition the Doppler-shifted hydrogen emissions. Tremendous progress has been made in the last decade in modeling and instrumental performance related to proton aurora. This is attested by the organization of two proton-dedicated workshops during the CEDAR meetings in 1994 by Prof. R. Smith and in 1999 by Dr. M. Galand. A special section in the Journal of Geophysical Research on “Proton precipitation into the atmosphere” followed, emphasizing to the ionospheric and magnetospheric communities the role the energetic protons play in the atmosphere [Galand, 2001]. In the present paper, we review the research conducted in the past 10 years on proton aurora and focus on the ground-based observations and analysis of the Hα (656.3 nm) and Hβ (486.1 nm) Balmer emissions. After a short description of the experimental sites used recently for proton aurora observations, we will first present photometric observations and thus discuss spectroscopic measurements and their analysis using comprehensive transport models. Finally, we provide recommendations for the future of this field.

1 2. COMMONLY USED EXPERIMENTAL SITES Proton precipitation is significant at the equatorial edge of the afternoon and evening auroral oval for moderate geomagnetic activity. Such a region tracking plasma sheet particle source can be probed from: - Poker Flat Research Range (PFRR), with soon the addition of the Advanced Modular Incoherent Scatter Radar (AMISR), - the site of the European Incoherent SCATter Radar (EISCAT) at Ramfjordmøen, near Tromsø, Norway (hereafter referred as the Tromsø station). The location of these two stations is given in Table I. Nighttime observation period extends from late August to beginning of May from PFRR and from early October to early April from Tromsø but excludes near-full moon periods. Dayside measurements in darkness conditions are possible from the auroral station in Adventdalen, Svalbard, due to its high geographic and geomagnetic latitudes (see Table I). The sun is well below the horizon (>8º) for more than two months around winter solstice. Observations from Svalbard include detection of the cusp and the Low-Latitude Boundary Layer (LLBL). Coordination with the near-by EISCAT Svalbard Radar (ESR) provides an assessment of the ionospheric state. Station Geographic latitude Geographic longitude Geomagnetic latitude Poker Flat, Alaska 65.1ºN 212.5ºE 65.2ºN Tromsø, Norway 69.6ºN 19.2ºE 66.4ºN Adventdalen, Svalbard 78.9ºN 15.8ºE 74.9ºN Table I: Experimental sites where NSF-funded observations of proton aurora have recently been carried out from the ground.

3. PHOTOMETRIC OBSERVATIONS 3.1 Instruments and Techniques The Meridian Scanning Photometer (MSP) has been extensively used to characterize the latitudinal extent and dynamics of the proton and electron aurora. Two such instruments have been operating at Poker Flat and at Svalbard (cf. Section 2). They consist of a set of spatially scanning telescopes with optical interference filters that tilt on and off the auroral wavelengths of interest so as to allow the removal of the background – including the contamination by intense electron aurora - from the signal. The filters commonly used have a bandwidth around 0.5 nm. The Hβ line is used for tracking the proton aurora. Even though the Hα line is brighter than the Hβ line, the close spectral proximity to the bright N2 1PG auroral emission and the OH(6,1) airglow emission makes it difficult to isolate. The other auroral wavelengths selected in the MSP are for identifying the hard + component (OI 557.7 nm and N2 1NG 427.8 nm) and the soft component (OI 630.0 nm, 844.6 nm, and OII 732.0 nm) of electron aurora. It should be noted that proton precipitation also contributes to these channels [e.g., Deehr and Lummerzheim, 2001; Lummerzheim et al., 2001]. The scan takes 16 s and is carried out along the North-South geomagnetic meridian covering the whole 180º angle range. The field of view of the instrument is 1º (larger for H emission). The data are usually presented as keograms showing the magnetic meridian profile in a given channel (after removal of the background) as a function of time. An archive of the data covering the last decade at Svalbard and the last two decades at Poker Flat can be found at: http://photon.gi.alaska.edu/home.html. The MSP at Svalbard is supported by NSF through University Alaska Fairbanks (UAF), in cooperation with University Centre on Svalbard (UNIS) and Norwegian funding sources. The MSP at Poker Flat, which was originally operating in support of

2 sounding rocket campaigns, has observational schedules developed over the years as a part of the NSF/CEDAR initiative [Deehr and Lummerhzeim, 2001]. The first simultaneous two-point comparison of proton aurora brightness was proposed by Nicholson et al. [2003] between MSPs of two auroral stations, Poker Flat, Alaska, and Gillam, Manitoba. This comparison demonstrates that the instruments produce qualitatively consistent measurements.

3.2 Scientific achievements Because of the diffuse nature of the H emissions and their relatively weak brightness compared with electron aurora, the use of H emissions as an ionospheric signature of proton precipitation and magnetopsheric processes has been slow to evolve compared with the well documented bright auroral arcs associated with hard electron precipitation. With the development of better interference filters and detector sensitivity, MSPs operating at Hβ have provided crucial information on the proton aurora over the past two decades. Research has thus focused on the relative morphology and dynamics of proton and electron aurora and on the identification of the optical, ionospheric signatures of magnetospheric regions and processes, especially during substorm periods. For reaching this goal, two approaches have commonly been applied. The first one is based on the deployment of ground-based chain of MSPs (and associated instruments, such as all-sky cameras) [e.g., Voronkov et al., 1999; Wanliss et al., 2000]. The second one appeals to a large number of observations from a given station [e.g., Deehr and Lummerzheim, 2001; Takahashi and Fukunishi, 2001 (with 2D imaging); Donovan et al., 2003]. We report here some of the recent findings: • A combination of all-sky imagers and MSP at Poker Flat was used to identify the optical signature of the auroral substorm phases in the Alaskan sector [Deehr and Lummerzheim, 2001]. During the growth phase, there are generally three precipitation regions separable in latitude, as illustrated in Figure 1: poleward arcs and bands associated with soft precipitation (E<1 keV), e- arc at the trapping boundary associated with hard precipitation (E~10 keV), and hydrogen, diffuse arc, extending over 100 km of latitude. Based on the study of a large number of cases, Deehr and Lummerzheim [2001] reported the separation and very different temporal behavior of the onset arc and the peak proton precipitation and concluded that the auroral substorm originates poleward of dipolar field lines and in the region of upward field-aligned current, which does not support theories of substorm onset that depend on resonance effects on closed, dipolar field lines [e.g., Samson et al., 1992].

Figure 1: Geographic latitude of the intensity maxima for three auroral arcs from PFRR meridian- scanning photometer record for February 1, 1990. The 557.7 nm and 630.0 nm OI were used for the electron-induced arcs (with lower values of the 630.0 nm/557.7 nm brightness ratio for the hard e-

arcs). The 486.1 nm Hβ emission was used for the H arc. The assumed altitude in all three cases was 110 km. Note that if a higher altitude were used for the less energetic electron onset arc the separation of the arcs would be even more apparent [from Deehr and Lummerzheim, 2001].

3 • Donovan et al. [2003] developed a simple algorithm to estimate the isotropy boundary from latitudinal profiles of the Hβ MSP channel. It corresponds to the equatorward boundary of the proton aurora, an important magnetospheric boundary that differentiates between regions of bounce trapping and strong pitch angle scattering. The knowledge of its location provides a good indication of the amount of stretching on the magnetotail. Applying the algorithm to a 10-year MSP dataset from Gillam, Manitoba, Donovan et al. [2003] demonstrated a strong correlation between the magnetic latitude of the ion isotropy boundary derived from MSP data and the inclination of the magnetic field as measured at GOES 8. Merging MSP Hβ data sets from two auroral stations (Poker Flat and Gillam), Nicholson et al. [2003] demonstrated that the most equatorward extent of the proton aurora oval is offset duskward from the midnight meridian by approximately 45 min and that the ion isotropy boundary is symmetric within 2 hours magnetic local time (MLT) of this point. • The pulsed proton events observed in the dayside aurora over Svalbard are attributed to the reconnection process in which magnetosheath enter the magnetosphere on newly interconnected field lines. The field lines will move in the direction of convection so that particles with different energies located on the same field line will reach the ionosphere dispersed in both space and time. The pulsed nature of the auroral H emission is similar in temporal and spatial changes to the discrete dayside electron aurora associated with flux transfer events at the magnetospheric boundary layer [Deehr et al., 1998; Lorentzen and Moen, 2000, and references therein]. MSP measurements have been used as complementary dataset. While spectroscopic observations yield quantitative information on the incident protons through the analysis of the Doppler profile of H emissions, they are carried out over a few-degree field of view. As such, MSP measurements provide a crucial context on the latitude extent of the proton and electron aurora [e.g., Sigernes et al., 1996a; Deehr et al., 1998; Lorenzten and Moen, 2000; Lanchester et al., 2003]. MSP data have also been used in support of rocket campaigns to facilitate the launch decision and identify the ionospheric signatures of particle precipitation along the flight path [e.g., Lorentzen et al., 1996].

4. SPECTROSCOPIC ANALYSIS 4.1 Instruments and Techniques

High resolution spectrographs have been used to acquire spectral profiles of Hα and Hβ emissions for quantitative analysis of the proton aurora. Ebert-Fastie spectrograph (EFS), scanning over the selected H Balmer spectral region, consist mainly of one large spherical mirror, one plane diffraction grating and a pair of slits [Fastie, 1952; Sivjee et al., 1979; Sigernes et al., 1996b]. When the grating turns, the image of the entrance slit is observed at the exit slit in different wavelengths, determined by the angular position of the grating [Sigernes et al., 1996a]. EFS have been deployed: - at Svalbard (operated by UAF/UNIS with partial support by NSF) [Sigernes et al., 1996a; Deehr et al., 1998; Lorentzen et al., 1998]. Two spectrographs (0.5 m and 1 m) measuring near Hα or Hβ were used for observing proton aurora. For a 1 mm slit, the Full Width Half Maximum (FWHM) of the H Balmer spectrograph is between 0.2 nm and 0.6 nm. The field of view is ~5°. Each wavelength scan takes approximately 12 s, but the processed, useable profile is the sum of scans within a 2 to 4-min time segment. - at Poker Flat (operated by UAF) [Lummerzheim and Galand, 2001]. The 1-m instrument near Hβ has a FWHM of 0.43 nm and a field of view of 7° around the magnetic zenith. A profile is

4 acquired each 16 s but up to 15 scans are averaged together to improve the signal to noise, yielding a time resolution of 4 min.

Application of CCD detectors to spectroscopy allows simultaneous registration of all spectral elements of a line profile resulting in a large sensitivity gain. Such imaging spectrographs are currently the optimal instruments for high spectral resolution measurements of auroral hydrogen lines [Baumgardner et al., 1993]. One such instrument, the High Throughput Imaging Echelle Spectrograph (HiTIES) developed at Boston University (BU), has been use in proton aurora campaigns. It is an imaging spectrograph capable of simultaneously observing multiple wavelengths in a wide spectral region from 390.0 to 770.0 nm [Chakrabarti et al., 2001]. HiTIES employs an Echelle grating, which is used at high orders and has a free spectral range of about 15 nm in a given order. Unlike conventional astronomical Echelle spectrographs, HiTIES uses a long input slit (45 mm) to increase its throughput. This instrument uses a “mosaic” of interference filters placed at the first image plane of the spectrograph to separate orders. This instrument has been deployed: - at Tromsø during two winter-long campaigns between 2001 and 2003 (deployed and operated by BU through a NSF/CEDAR grant) [Galand et al., 2003; Galand et al., 2004]. The spectral resolution in the Hα and Hβ windows (FWHM) is 0.10 nm and 0.06 nm, respectively. The field of view of the processed profiles is 2º and the temporal resolution is 4 min. - at Svalbard since 2000 (operated by University of Southampton, UK, in automatic mode; partial support from NSF through modeling efforts) [Lanchester et al., 2003; Ivchenko et al., 2004]. The Balmer line selected is Hβ with a FWHM of 0.13 nm. The field of view is 8º. The integration time can be as low as 10 s, but the temporal resolution of the processed proton aurora data is larger, of the order of the minute. Information on the experimental sites can be found in Section 2. All the quantitative studies carried out with these instruments are based on observations obtained with a viewing centered on the magnetic zenith. With such a configuration, the downward component of the precipitation is separated from the upward component through Doppler shift. In addition, this configuration minimizes any significant contribution of different proton sources in the magnetosphere to the H profile. The wavelength calibration of the spectrograph in the Hα and Hβ windows is commonly performed using spectral lamps (hydrogen, neon, xenon, argon, krypton, rubidium). The photometric calibration is carried out using two different light sources: one whose absolute calibration is precisely known (e.g., C14 source) and the other, cross-calibrated to the first one, with a large spectral range, and easily transportable to the experimental site (e.g., Tungsten lamp) [Galand et al., 2004].

4.2 Transport models The effort invested in auroral proton studies has led to the development of more comprehensive models describing the transport of an energetic incident proton beam in the upper atmosphere, its energy degradation and scattering, and its partial conversion to H atoms through charge-changing reactions. Several theoretical models of H+/H atom transport in space have been developed recently, based on: 1D or 3D-in-space Monte Carlo method [Kozelov, 1993; Lorentzen et al., 1998; Synnes et al., 1998; Gérard et al., 2000; Lorentzen, 2000; Solomon, 2001; Fang et al., 2004] or an explicit solution of the coupled proton/H atom Boltzmann equations along the magnetic field line [Basu et

5 al., 1993; Galand et al., 1998; Basu et al., 2001]. Given the energy and pitch angle distribution of the incident proton flux at the top of the atmosphere (~600-800 km), the altitude profile of the number density of the atmospheric species, and the H+/H impact cross sections on the atmospheric species, these steady-state models compute the proton and H atom fluxes over a range of altitudes, energies, and pitch angles. The Doppler profiles of H emissions along the magnetic zenith can be easily derived from these fluxes [e.g., Lanchester et al., 2003]. To date, only two of these models [Lorentzen et al., 1998 and Galand et al., 1998] have been used for the analysis of spectroscopic observations of the H Balmer lines in proton aurora. The first one is a 3D-in-space model including the spreading of the incident beam but neglecting collisional scattering, which modifies the shape of the H emission profile. The second is a 1D-in-space model solving the transport along the magnetic field line and including scattering of magnetic and collisional origins.

4.3 Data analysis and findings Analysis of high spectral resolution Doppler profiles of H emissions observed in proton aurora using comprehensive H+/H transport models has yielded new findings in the past decade: • Qualitative and quantitative comparisons of simultaneous and co-located multi-instrument observations of proton aurora (electron density, H Balmer brightnesses and spectra, and in situ particle flux at the top of the atmosphere) attest to the capability of high resolution spectrographs to detect the presence of proton precipitation, to assess the characteristics of the incident proton flux, and to confirm the role of proton precipitation as a significant source of ionization in the high latitude ionosphere [Galand et al., 2003; Lanchester et al., 2003]. • The shape of the spectral profiles of H emissions observed from the ground is blue- shifted, as most of the emitting H atoms are propagating downward (as illustrated in Figure 2). The shape of the violet (blue-shifted) wing, rather than the location of the peak of the blue-shifted Hβ line profile, is a suitable indicator of the mean energy of the precipitating proton flux – with the shape of the energy and pitch angle distribution of the incident protons as input [Lummerzheim and Galand, 2001; Lanchester et al., 2003]. At low spectral (> 1 nm) resolution, the Doppler shift of the peak is affected by the violet wing and the red-shifted wing is explained by the instrumental line broadening [Galand et al., 1998]. At high spectral resolution (≤ 0.4 nm), the red-shifted wing is distinguishable from the instrumental line broadening and is evidence of back-scattered hydrogen atoms; the observed red-shifted wing is then well reproduced by modeling through collisional angular redistribution [Lummerzheim and Galand, 2001; Lanchester et al., 2003; Galand et al., 2004]. • A spectral resolution of 0.1 nm is sufficient to identify and take into account contaminating airglow OH emission, auroral N2 1PG emission, and geocoronal and galactic Hα contribution in Hα spectral profiles observed in proton aurora [Galand et al., 2004]. This finding opens the possibility of using the strongest Balmer line for probing the proton aurora.

• The Balmer decrement, defined as the ratio of the total brightness of the Hα to Hβ lines in proton aurora, has been the focus of both experimental and theoretical studies for the last 40 years. This ratio could, in principle, yield information on the spectral characteristics of

6 the incoming protons [Sigernes et al., 1996a]. However, numerous uncertainties in measured and modeled Hα and Hβ line profiles still preclude using the Balmer decrement as an indicator of the precipitating proton flux [Galand et al., 2004]. • On the nightside, the spectral profiles of H emissions observed from the ground are strongly asymmetric about the blue-shifted peak with a large violet wing. By comparison, the H emission profiles measured at dayside are narrower with a violet wing of smaller spectral extent, as illustrated in Figure 2 [Galand et al., 2004 and references therein]. Deehr et al. [1998] observed a more pronounced variation in the wavelength of the peak of the Doppler- shifted line at dayside and a nearly symmetrical shape for the spectral profile. They interpret this fact as an indication that the dayside proton aurora is dominated by mono-energetic or narrow Gaussian energy spectra of the precipitating protons, which is consistent with low- latitude satellite observations. Modeling of mono-energetic precipitation by Lorentzen et al. [1998] supports this hypothesis. Dayside proton precipitation is likely to be monoenergetic owing to the “velocity filter” resulting from dayside reconnection and convection [Lockwood and Smith, 1992; Onsager and Elphic, 1996, and references therein]. • The H emission spectrum is a direct measure of the reconnection processes at the dayside magnetopause through the detection of the temporal variation of the ion energy at a given location [Deehr et al., 1998; Lorentzen and Moen, 2000]. • 3D modeling of the H+/H transport shows that the highest energy ions could be separated spectroscopically from the lowest energy population, assuming a precipitation region expanding over 75 km. Therefore, the velocity filter effect should be detectable [Deehr et al., 1998].

Figure 2: (Left) Hβ spectral profile in proton aurora observed at nightside from Tromsø (solid line). The dotted line represents the decontaminated Doppler-shifted profile and the dashed line, the modeled profile obtained with a Maxwellian distribution in energy and a mean energy of 40 keV. The H lamp spectrum has been overplotted for reference (narrow line near Hβ). The vertical solid line corresponds to the H Balmer line at rest [after Galand et al., 2004] (Right) Hβ spectral profile in proton aurora observed at dayside from Svalbard (solid line). The dashed line is the modeled profile obtained from a proton /H atom transport code using as input an ion flux measured simultaneously by DMSP over Svalbard. The vertical dotted line corresponds to the H Balmer line at rest [after Lanchester et al., 2003].

7 5. CONCLUSION and RECOMMENDATIONS Since ground-based instruments make continuous observations at one location, they are well suited to study the variability and temporal evolution of the aurora. The Doppler-shifted H emissions are the spectroscopic signature of energetic proton precipitation. They are used for imaging the proton aurora, an excellent probe for tracking magnetospheric processes and regions and for assessing the energy input to the ionosphere. While MSPs provide the morphology and dynamics of the proton aurora, the combination of high resolution measurements with modeling provides a method of estimating the incoming energy and changes in flux of precipitating protons, being given the shape of the energy and pitch angle distributions of the incoming particles. To describe the interaction of protons with the upper atmosphere in a quantitative manner requires good spectral resolution (≤ 0.1 nm for Hα and ≤ 0.4 nm for Hβ) and appropriate temporal resolution (≤ 4 min). There follows here a set of issues and recommendations for proton aurora studies in the near future: 9 Large-scale SUBSTORM studies: o The magnetospheric mechanism associated with the substorm onset is still under debate (as discussed by Deehr and Lummerzheim [2001]). o The relative extent of proton and electron aurora, studied during the substorm period, has only been carried out over a limited MLT sector. A larger coverage in MLT - along with magnetic latitude coverage and magnetically conjugated observations - would better constrain our ability to apprehend the magnetospheric configuration and evolution. o Multi-point measurements of the ion isotropy boundary (as illustrated by Nicholson et al. [2003] using two auroral stations) will enhance our understanding of the topology and dynamics of the inner magnetosphere. Æ Development, validation, and deployment of 2D, multicolor, all-sky imaging system operating simultaneously at Hβ emission line and its background, as tested at Syowa Station, Antarctica [Takahashi and Fukunishi, 2001] and at Poker Flat [Lummerzheim et al., 2003]. The optimum system would be a 3D imaging system (2D in space and 1D in wavelength). Æ Deployment of a chain of optical instruments probing the H Balmer lines aimed to cover a large range of MLT, a range of auroral latitudes along a magnetic meridian, or magnetically conjugated regions. The instrument set includes 1D and 2D imaging systems, and high resolution spectrographs.

9 QUANTITATIVE ANALYSIS of the proton aurora: o While the shape of the dayside Hβ profiles in proton aurora is well understood [Lanchester et al., 2003], the violet wing of the observed H Balmer profiles at nightside is underestimated by models [Galand et al., 2004]. o The brightness of the H Balmer lines observed both dayside and nightside sectors is not well reproduced by models, even when the latter are driven by combined particle measurements [Lorentzen et al.¸1998; Galand et al., 2003]. o The brightness ratio observed between Hα and Hβ lines in proton aurora is not explained by modeling [Galand et al., 2004]. o We need to improve our ability to infer the ionospheric response to proton precipitation from the analysis of H Balmer profiles.

8 Æ Need for laboratory measurements of: + ¾ differential cross section of H and H impact (on N2 and O2 at large scattering angles, and on O over the whole scattering range) ¾ Balmer emission cross section (Hβ above 3 keV, H Balmer on O) Æ Estimation of the contribution of atmospheric scattering on the H profiles yielding extinction due to scattering out of the field of view and contamination from scattering into the field of view: ¾ Observations of the proton aurora in two or more directions at high spectral resolution at a given location ¾ Development of 2D imaging system (as describe above) ¾ Application of radiative transfer model to the H emission produced in proton aurora Æ Assessment of the spreading of the beam on the H spectral profiles using 3-D models (e.g., Fang et al. [2004] and references therein) Æ Further combined experiment between incoherent scatter radars, high resolution spectrographs near H Balmer lines, and in situ particle measurements from satellites or rockets (over few 10s eV to several 100s of keV)

9 DAYSIDE proton aurora: o The ion energy distribution observed from one station reflects the instantaneous reconnection rate near the meridian of the observation. Observations in several directions from one or more stations will determine the azimuthal extent on the magnetopause of the reconnection region and the direction of the convection in the ionosphere [Deehr et al., 1998] Æ Observation of the velocity filter in the cusp region by a chain of high spectral resolution spectrographs at different geomagnetic latitudes

Æ Extension of the H Balmer (especially Hα) spectroscopic observations to sunlit conditions

Acknowledgements: We are very grateful to C. Deehr, B. Lanchester, and D. Lummerzheim for their careful reading and enriching comments on the manuscript. M.G. is supported by NSF grant ATM-0003175.

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