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Investigation of Polar Mesosphere Summer Echoes in Northern Scandinavia
Umeå University, Faculty of Science and Technology, Physics.
2003 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This PhD thesis deals with phenomena which are closely related to the unique thermal structure of the polar summer mesosphere, namely Polar Mesosphere Summer Echoes (PMSE). PMSE are strong radar echoes commonly observed by VHF MST radars from thin layers in the 80-90 km altitude interval at high latitudes during summer. They follow a seasonal pattern of abrupt appearance in late May and a gradual disappearance in mid-August. This period corresponds roughly to the time between the completion of the summer time cooling of the polar mesopause to the time of reversal of the mesospheric circulation to autumn condition. In this connection, PMSE are associated with the extremely low temperatures, i.e. below 140 K, which are unique to the polar summer mesopause. Traditional theories of radar (partial) reflection and scattering have been unable to explain the PMSE and the exact mechanism for their occurrence remains unclear despite the steadily increasing interest in them over the past 20 years. Currently accepted theories regarding the mechanism giving rise to PMSE agree that one of the conditions needed for enhanced radar echoes is the presence of low-mobility charge carries such as large cluster ions and ice aerosols which capture the ambient electrons. It has been established that the PMSE are in some way associated with noctilucent clouds (NLC), layers of ice crystals, which constitute the highest observed clouds in the earth’s atmosphere. PMSE occurrence and dynamics are also found to be closely connected with the planetary and gravity waves.

Observations of PMSE presented in this thesis have been carried out by the Esrange MST radar (ESRAD) located at Esrange (67°56’N, 21°04’E) just outside Kiruna in northernmost Sweden. The radar operates at 52 MHz with 72 kW peak power and a maximum duty cycle of 5%. The antenna consists of 12x12 array of 5-element Yagis with a 0.7l spacing. During the PMSE measurements the radar used a 16-bit complementary code having a baud length of 1mS. This corresponds to height resolution of 150 m. The sampling frequency was set at 1450 Hz. The covered height range was 80-90 km. The presence of PMSE was determined on the basis of the radar SNR (signal-to-noise ratio). The PMSE measurements have been made during May-August each year since 1997.

PMSE seasonal and diurnal occurrence rates as well as dynamics have been studied in connection with tidal winds, planetary waves, temperature and water vapor content in the mesosphere (Papers I, IV and VI). Simultaneous and common-volume observations of PMSE and noctilucent clouds have been performed by radar, lidar and CCD camera (Paper V). Correlation between variations in PMSE and variations in extra ionization added by precipitating energetic electrons or high-energy particles from the Sun has been examined (Papers II and III). Possible influence of transport effects due to the electric field on PMSE appearance has been studied during a solar proton event (Paper III).

Place, publisher, year, edition, pages
Umeå: Fysik , 2003. , 36 p.
Series
IRF Scientific Report, ISSN 0284-1703 ; 283
Keyword [en]
Physics, Polar Mesosphere Summer Echoes, middle atmospheric dynamics, ionospheric disturbances, solar radiation, aerosol, MST radar, planetary waves, noctilucent clouds
Keyword [sv]
Fysik
National Category
Physical Sciences
Research subject
Space Physics
Identifiers
URN: urn:nbn:se:umu:diva-176ISBN: 91-7305-563-8 (print)OAI: oai:DiVA.org:umu-176DiVA: diva2:141940
Public defence
2004-02-13, Umeå, 10:00
Available from: 2003-12-23 Created: 2003-12-23Bibliographically approved
List of papers
1. Investigations of the possible relationship between PMSE and tides using a VHF MST radar
Open this publication in new window or tab >>Investigations of the possible relationship between PMSE and tides using a VHF MST radar
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1998 In: Geophysical Research Letters, Vol. 25, no 17, 3297-3300 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:umu:diva-3316 (URN)
Available from: 2003-12-23 Created: 2003-12-23Bibliographically approved
2. Are variations in PMSE intensity affected by energetic particle precipitation?
Open this publication in new window or tab >>Are variations in PMSE intensity affected by energetic particle precipitation?
2002 In: Annales Geophysicae, Vol. 20, 539-545 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:umu:diva-3317 (URN)
Available from: 2003-12-23 Created: 2003-12-23Bibliographically approved
3. Polar Mesosphere Summer Echoes during the July 2000 Solar Proton Event
Open this publication in new window or tab >>Polar Mesosphere Summer Echoes during the July 2000 Solar Proton Event
2004 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 22, 759-771 p.Article in journal (Refereed) Published
Abstract [en]

The influence of the solar proton event (SPE) 14–16 July 2000 on Polar Mesosphere Summer Echoes (PMSE) is examined. PMSE were observed by the Esrange VHF MST Radar (ESRAD) at 67°53'N, 21°06'E. The 30MHz Imaging Riometer for Ionospheric Studies IRIS in Kilpisjärvi (69°30'N, 20°47'E) registered cosmic radio noise absorption caused by ionisation changes in response to the energetic particle precipitation. An energy deposition/ion-chemical model was used to estimate the density of free electrons and ions in the upper atmosphere. Particle collision frequencies were calculated from the MSISE-90 model. Electric fields were calculated using conductivities from the model and measured magnetic disturbances. The electric field reached a maximum of 91mV/m during the most intensive period of the geomagnetic storm accompanying the SPE. The temperature increase due to Joule and particle heating was calculated, taking into account radiative cooling. The temperature increase at PMSE heights was found to be very small.

The observed PMSE were rather intensive and extended over the 80–90km height interval. PMSE almost disappeared above 86km at the time of greatest Joule heating on 15 July 2000. Neither ionisation changes, nor Joule/particle heating can explain the PMSE reduction. Transport effects due to the strong electric field are a more likely explanation.

Place, publisher, year, edition, pages
European Geosciences Union (EGU), 2004
Keyword
Meteorology and atmospheric dynamics (middle atmospheric dynamics), ionosphere (ionospheric disturbances; solar radiation and cosmic ray effects)
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:umu:diva-3318 (URN)10.5194/angeo-22-759-2004 (DOI)
Available from: 2003-12-23 Created: 2003-12-23 Last updated: 2012-05-14Bibliographically approved
4. The 1997 PMSE season - its relation to wind, temperature and water vapor
Open this publication in new window or tab >>The 1997 PMSE season - its relation to wind, temperature and water vapor
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1998 In: Geophysical Research Letters, Vol. 25, no 11, 1867-1870, p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:umu:diva-3319 (URN)
Available from: 2003-12-23 Created: 2003-12-23Bibliographically approved
5. Polar mesosphere summer echoes and noctilucent clouds: Simultaneous and common-volume observations by radar, lidar and CCD camera
Open this publication in new window or tab >>Polar mesosphere summer echoes and noctilucent clouds: Simultaneous and common-volume observations by radar, lidar and CCD camera
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2000 In: Geophysical Research Letters, Vol. 27, no 5, 661-664 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:umu:diva-3320 (URN)
Available from: 2003-12-23 Created: 2003-12-23Bibliographically approved
6. Noctilucent clouds, PMSE and 5-day planetary waves: a case study
Open this publication in new window or tab >>Noctilucent clouds, PMSE and 5-day planetary waves: a case study
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2002 In: Geophysical Research Letters, 29, 10, 10.1029/2001Gl014022, 1-4, 2002., Vol. 29, no 10, 1-4 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:umu:diva-3321 (URN)
Available from: 2003-12-23 Created: 2003-12-23Bibliographically approved

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